Work vehicle, monitoring system for work vehicle, and tracked work vehicle

ABSTRACT

A work vehicle includes a rotary member, a support member, a sealing ring, a pressure controller, and a vehicle speed determination component. The rotary member has a first hydraulic fluid supply channel to supply the hydraulic fluid to the steering clutch, and is rotated by power from the transmission when the steering clutch is engaged. The sealing ring is disposed between the rotary member and the support member and is mounted adjacent to the connected part between the first hydraulic fluid supply channel and the second hydraulic fluid supply channel. The pressure controller controls the engagement pressure to be a specific first pressure when the vehicle speed is determined not to be equal to or greater than a specific speed, and controls the engagement pressure to decrease from the first pressure when the vehicle speed is determined to be equal to or greater than a specific speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2016/055945, filed on Feb. 26, 2016. This U.S.National stage application claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2015-056198, filed in Japan on Mar. 19,2015, 2015-056199 filed in Japan on Mar. 19, 2015, and 2015-056200,filed in Japan on Mar. 19, 2015 the entire contents of which are herebyincorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a work vehicle, a monitoring system fora work vehicle, and a tracked work vehicle.

Description of the Related Art

With a work vehicle that moves on crawler belts, such as a bulldozer,the power of the engine is transmitted through a transmission to theleft and right drive wheels, and the left and right crawler belts aredriven by these drive wheels. A bulldozer such as this is provided withsteering clutches and steering brakes corresponding to the left andright drive wheels. Left and right turns are executed by hydraulicallycontrolling the left and right steering clutches and steering brakes(see Japanese Laid-Open Patent Application 2013-231324, for example).

With the steering clutches and steering brakes in Japanese Laid-OpenPatent Application 2013-231324, a low hydraulic pressure state resultsin an engaged state, while a state in which the hydraulic pressure isequal to or greater than a specific level results in a disengaged state.

SUMMARY Problem to be Solved by the Invention

With a steering clutch, rotary members on both sides connected to theclutch rotate during vehicle travel. Therefore, hydraulic fluid must besupplied to the clutch through at least one of the rotary members. Thatis, a hydraulic fluid supply channel has to be provided for supplyinghydraulic fluid to the rotary member from a support member that supportsthe rotary member. In this case, it is necessary to provide a sealingring to prevent the hydraulic fluid from seeping out at the connectedpart of the hydraulic fluid supply channel between the support memberand the rotary member.

The steering clutch in Japanese Laid-Open Patent Application 2013-231324has a mechanism such as a spring for pressing on a clutch plate duringengagement, and this results in a complicated clutch structure.Accordingly, there is a need for a more compact clutch that can applypressure by hydraulics alone. With a clutch such as this, it isdisengaged when no hydraulic pressure is being exerted, and engaged whenhydraulic pressure is exerted. Therefore, when a clutch such as this isused, something must be done to keep the PV value of the sealing ringfrom reaching the maximum permissible value.

This Specification discloses a work vehicle having a steering clutchthat is engaged in a state in which hydraulic pressure is being exerted,and a work vehicle with which the PV value of the sealing ring will notreach the maximum permissible value.

Means for Solving Problem

(A) The work vehicle according to the first mode includes an engine, atransmission, a steering clutch, a rotary member, a drive unit, asupport member, a sealing ring, a pressure controller, and a vehiclespeed determination component. The transmission changes the speed of therotary motion of the engine. The steering clutch transmits or cuts offpower from the transmission, and is engaged when supplied with hydraulicfluid that is under engagement pressure. The rotary member has a firsthydraulic fluid supply channel for supplying the hydraulic fluid to thesteering clutch, and is rotated by power from the transmission when thesteering clutch is engaged. The sealing ring is disposed between therotary member and the support member and is mounted adjacent to theconnected part between the first hydraulic fluid supply channel and thesecond hydraulic fluid supply channel.

The pressure controller controls the pressure of the hydraulic fluidsupplied to the steering clutch. The vehicle speed determinationcomponent determines whether or not the vehicle speed is equal to orgreater than a specific speed. The pressure controller controls theengagement pressure to a specific first pressure when the vehicle speedis determined not to be equal to or greater than the specific speed. Thepressure controller performs control to reduce the pressure of thehydraulic fluid from the first pressure when the vehicle speed isdetermined to be equal to or greater than the specific speed.

The work vehicle preferably further includes a rotational speed sensorthat measures the rotational speed of the sealing ring. The vehiclespeed determination component preferably determines the vehicle to beequal to or greater than the specific speed when the rotational speedmeasured by the rotational speed sensor is more than a specificrotational speed threshold.

The pressure controller preferably performs control to reduce thepressure so that the product of the rotational speed and the pressure ofthe hydraulic fluid does not exceed a specific upper limit when therotational speed measured by the rotational speed sensor is more thanthe rotational speed threshold.

The pressure controller preferably performs control to set the pressureof the hydraulic fluid to be the quotient of dividing a first product,which is the product of the first pressure and the rotational speedthreshold, by the rotational speed when the rotational speed is morethan the rotational speed threshold.

The work vehicle preferably further includes a gear sensor that sensesthe gear of the power transmission device that has been set by operatorinput. The vehicle speed determination component preferably determinesthe vehicle speed to be equal to or greater than the specific speed whenthe gear is a specific first gear or a gear higher than the first gear.

The pressure controller preferably performs control so that the pressureof the hydraulic fluid is set to a second pressure that is lower thanthe first pressure, when the gear is a first gear or a gear higher thanthe first gear.

The sealing ring preferably seals the gap between the rotary member andthe support member. The connected part is preferably formed by the gapsealed by the sealing ring.

(B) With a work vehicle that moves on crawler belts, such as abulldozer, the power of the engine is transmitted through a transmissionto left and right drive wheels, and the left and right crawler belts aredriven by these drive wheels. With a bulldozer such as this, steeringclutches and steering brakes are provided corresponding to the left andright drive wheels. Left and right turns are executed by hydraulicallycontrolling the left and right steering clutches and steering brakes(see Japanese Laid-Open Patent Application 2013-231324, for example).

With the steering clutches and steering brakes Japanese Laid-Open PatentApplication 2013-231324, they are engaged when the hydraulic pressure islow, and disengaged when the hydraulic pressure is equal to or greaterthan a specific level.

With a steering clutch, rotary members on both sides connected to theclutch rotate during vehicle travel. Therefore, hydraulic fluid must besupplied to the clutch through at least one of the rotary members. Thatis, a hydraulic fluid supply channel has to be provided for supplyinghydraulic fluid to the rotary member from a support member that supportsthe rotary member. In this case, it is necessary to provide a sealingring to prevent the hydraulic fluid from leaking out at the connectedpart of the hydraulic fluid supply channel between the support memberand the rotary member.

The steering clutch in Japanese Laid-Open Patent Application 2013-231324has a mechanism such as a spring for pressing on a clutch plate duringengagement, and this results in a complicated clutch structure.Accordingly, there is a need for a more compact clutch that can applypressure by hydraulics alone. With a clutch such as this, it isdisengaged when no hydraulic pressure is being exerted, and engaged whenhydraulic pressure is exerted. Also, when a clutch such as this is used,the surface pressure exerted on the sealing ring rises during rotationof the rotary member. Consequently, there is more wear to the sealingring when the sealing ring is used for an extended period. Therefore, itis helpful to be able to notify a worker when it is time to replace thesealing ring (maintenance information), according to the usage state ofthe sealing ring.

This Specification discloses a work vehicle monitoring system with whicha worker can be properly notified of maintenance information about thesealing ring, according to the usage state of the sealing ring in a workvehicle having a hydraulic clutch that is engaged in a state in whichhydraulic pressure is being exerted, as well as a work vehicle withwhich the notification is possible.

The monitoring system according to a first mode includes a work vehicleand a monitoring device that is provided on the exterior of the workvehicle and is capable of inputting information from the work vehicle.The work vehicle includes an engine, a transmission, a steering clutch,a rotary member, a drive unit, a support member, a sealing ring, arotational speed sensor, a controller, and an external output component.

The transmission changes the speed of the rotary motion of the engine.The steering clutch transmits or cuts off power from the transmission,and is engaged when supplied with hydraulic fluid. The rotary member hasa first hydraulic fluid supply channel for supplying the hydraulic fluidto the steering clutch, and is rotated by power from the transmissionwhen the steering clutch is engaged. The drive unit is driven by therotary member. The support member has a second hydraulic fluid supplychannel for supplying hydraulic fluid to the first hydraulic fluidsupply channel, and rotatably supports the rotary member. The sealingring is disposed between the rotary member and the support member and ismounted adjacent to the connected part between the first hydraulic fluidsupply channel and the second hydraulic fluid supply channel. Therotational speed sensor senses the rotational speed of the rotarymember.

The controller decides the pressure of the hydraulic fluid inside thefirst hydraulic fluid supply channel and the second hydraulic fluidsupply channel, and performs control so that the pressure of thehydraulic fluid is the decided pressure. The external output componentoutputs determination basis data related to the decided pressure, therotational speed, and the time controlled by the decided pressure whilethe rotary member is rotating, in a format that can be inputted by themonitoring device.

The monitoring device includes an output component. The output componentoutputs maintenance information about the sealing ring when thepredicted wear amount of the sealing ring obtained from thedetermination basis data exceeds a specific threshold.

The work vehicle according to a second mode is a work vehicle capable ofoutputting information to an external monitoring device, and includes anengine, a transmission, a steering clutch, a rotary member, a driveunit, a support member, a sealing ring, a rotational speed sensor, acontroller, and an external output component.

The transmission changes the speed of the rotary motion of the engine.The steering clutch transmits or cuts off power from the transmission,and is engaged when supplied with hydraulic fluid. The rotary member hasa first hydraulic fluid supply channel for supplying the hydraulic fluidto the steering clutch, and is rotated by power from the transmissionwhen the steering clutch is engaged. The drive unit is driven by therotary member. The support member has a second hydraulic fluid supplychannel for supplying the hydraulic fluid to the first hydraulic fluidsupply channel, and rotatably supports the rotary member. The sealingring is disposed between the rotary member and the support member and ismounted adjacent to the connected part between the first hydraulic fluidsupply channel and the second hydraulic fluid supply channel. Therotational speed sensor senses the rotational speed of the rotarymember.

The controller decides the pressure of the hydraulic fluid inside thefirst hydraulic fluid supply channel and the second hydraulic fluidsupply channel, and performs control so that the pressure of thehydraulic fluid is the decided pressure. The external output componentoutputs determination basis data related to the decided pressure, therotational speed, and the time controlled by the decided pressure whilethe rotary member is rotating, in a format that can be inputted by themonitoring device.

The above-mentioned determination basis data is preferably the valueobtained by integration of the product of the decided pressure and therotational speed at the same time as the decided pressure.

The above-mentioned external output component preferably outputsdetermination basis data to a communication line or to a removablestorage medium that can be written to by the work vehicle and that canbe read by the monitoring device.

The work vehicle according to a third mode includes an engine, atransmission, a steering clutch, a rotary member, a drive unit, asupport member, a sealing ring, a rotational speed sensor, and acontroller.

The transmission changes the speed of the rotary motion of the engine.The steering clutch transmits or cuts off power from the transmission,and is engaged when supplied with hydraulic fluid. The rotary member hasa first hydraulic fluid supply channel for supplying the hydraulic fluidto the steering clutch, and is rotated by power from the transmissionwhen the steering clutch is engaged. The drive unit is driven by therotary member. The support member has a second hydraulic fluid supplychannel for supplying the hydraulic fluid to the first hydraulic fluidsupply channel, and rotatably supports the rotary member. The sealingring is disposed between the rotary member and the support member and ismounted adjacent to the connected part between the first hydraulic fluidsupply channel and the second hydraulic fluid supply channel. Therotational speed sensor senses the rotational speed of the rotarymember.

The controller decides the pressure of the hydraulic fluid inside thefirst hydraulic fluid supply channel and the second hydraulic fluidsupply channel, and performs control so that the pressure of thehydraulic fluid is the decided pressure. The controller outputsmaintenance information about the sealing ring when the predicted wearamount of the sealing ring, obtained from the decided pressure, therotational speed, and the time controlled by the decided pressure whilethe rotary member is rotating, exceeds a specific threshold.

The controller preferably performs control to set the pressure to aspecific first pressure when the rotational speed measured by therotational speed sensor is equal to or less than a specific rotationalspeed threshold. The controller preferably performs control to reducethe pressure from the first pressure so that the product of therotational speed and the pressure of the hydraulic fluid will not exceeda specific upper limit when the rotational speed measured by therotational speed sensor is more than the specific rotational speedthreshold.

The controller preferably performs control to set the pressure of thehydraulic fluid to be the quotient of dividing a first product, which isthe product of the first pressure and the rotational speed threshold, bythe rotational speed when the rotational speed is more than therotational speed threshold.

The sealing ring preferably seals the gap between the rotary member andthe support member. The connected part is preferably formed by the gapsealed by the sealing ring.

With the monitoring system according to the first mode, the work vehicledecides the pressure of the hydraulic fluid inside the hydraulic fluidsupply channels, and outputs determination basis data related to thedecided pressure, the rotational speed, and the time controlled by thedecided pressure while the rotary member is rotating, in a format thatcan be inputted by the monitoring device. The monitoring device outputsmaintenance information about the sealing ring based on the predictedwear amount of the sealing ring obtained from the determination basisdata. Thus, a worker is properly notified about replacement of thesealing ring and so forth according to the usage state of the sealingring.

With the work vehicle according to the second mode, the work vehicledecides the pressure of the hydraulic fluid inside the hydraulic fluidsupply channels, and outputs determination basis data related to thedecided pressure, the rotational speed, and the time controlled by thedecided pressure while the rotary member is rotating, in a format thatcan be inputted by the monitoring device. Therefore, the monitoringdevice can properly notify a worker about replacement of the sealingring and so forth according to the usage state of the sealing ring, onthe basis of the determination basis data.

With the work vehicle according to the third mode, the work vehicledecides the pressure of the hydraulic fluid inside the hydraulic fluidsupply channels, and outputs maintenance information about the sealingring based on the predicted wear amount of the sealing ring obtainedfrom the decided pressure, the rotational speed, and the time controlledby the decided pressure while the rotary member is rotating. Thus, thework vehicle can properly notify a worker about replacement of thesealing ring and so forth according to the usage state of the sealingring.

(C) With a tracked work vehicle that moves on crawler belts, such as abulldozer, the power of the engine is transmitted through a transmissionto left and right drive wheels, and the left and right crawler belts aredriven by these drive wheels. With a bulldozer such as this, steeringclutches and steering brakes are provided corresponding to the left andright drive wheels. Left and right turns are executed by hydraulicallycontrolling the left and right steering clutches and steering brakes(see Japanese Laid-Open Patent Application 2013-231324, for example).

A tracked work vehicle sometimes performs work in which a high load isexerted on one or both of the crawler belts, such as in boulder dozingwork, for example. This can result in slippage of the steering clutch,causing an excessively high load to be exerted. Japanese Laid-OpenPatent Application 2013-231324 discloses a method in which, in order toprotect the left and right steering clutches, steering clutch slippageis sensed, and if the amount of slippage is large, either the engineoutput is reduced or the clutch is forcibly released.

With the method in Japanese Laid-Open Patent Application 2013-231324,however, the excavating force of the blade decreases when a high load isexerted on one or both crawler belts, and this diminishes workefficiency.

This Specification discloses a tracked work vehicle with which there isless of a drop in work efficiency when a high load is exerted on one orboth crawler belts.

The tracked work vehicle according to a first mode includes an engine,left and right drive units, a power transmission device, left and rightsteering clutches, left and right steering brakes, a stall statedetermination component, and a steering clutch controller. The left andright drive units respectively have left and right crawler belts andleft and right drive wheels for driving the left and right crawlerbelts. The power transmission device transmits the power of the engineto the left and right drive wheels. The left and right steering clutchesare respectively disposed between the power transmission device and theleft and right drive wheels and transmit or cut off power. The left andright steering brakes are respectively disposed between the left andright steering clutches and the left and right drive wheels, andrespectively brake the rotation to the left and right drive wheels.

The stall state determination component determines whether or not theleft and/or right drive wheel is in a stall state. The steering clutchcontroller controls the clutch pressures of the left and right steeringclutches. The steering clutch controller controls the clutch pressuresof the left and right steering clutches to be a first pressure when itis determined that the left and right drive wheels are not in a stallstate. If the left and/or right drive wheel is determined to be in astall state, the clutch pressure of the steering clutch corresponding tothe drive wheel determined to be in a stall state is raised to a secondpressure that is higher than the first pressure.

The tracked work vehicle preferably includes a gear sensor, a firstrotational speed sensor, and a second rotational speed sensor. The gearsensor preferably senses the gear of the power transmission device setby the operator. The first rotational speed sensor preferably senses afirst rotational speed, which is the rotational speed of the input shaftof the power transmission device. The second rotational speed sensorpreferably senses a second rotational speed, which is the rotationalspeed of the output shaft of the power transmission device.

The stall state determination component preferably includes a speedratio computer and a vehicle speed sensor. The speed ratio computerpreferably computes the speed ratio, which is the ratio of the secondrotational speed to the first rotational speed. The vehicle speed sensorpreferably senses the speed of the tracked work vehicle. The stall statedetermination component preferably determines the left and right drivewheels to be in a stall state when the gear sensed by the gear sensor isfirst gear, the speed ratio is equal to or less than a first speedratio, and the vehicle speed is equal to or less than a specific firstspeed.

Preferably, the drive torque exerted on the steering clutch when thegear sensed by the gear sensor is first gear, the speed ratio is thefirst speed ratio, and the vehicle speed is the first speed, is equal toor less than the clutch capacity of the steering clutch when the firstpressure is exerted on the steering clutch.

The work vehicle preferably includes a third rotational speed sensor anda fourth rotational speed sensor. The third rotational speed sensorpreferably senses a third rotational speed, which is the rotationalspeed of the input-side rotary members of the left and right steeringclutches. The fourth rotational speed sensor preferably senses a fourthrotational speed, which is the rotational speed of the output-siderotary members of the left and right steering clutches. The stall statedetermination component includes a speed differential computer thatcomputes the speed differential between the third rotational speed andthe fourth rotational speed for each of the left and right steeringclutches.

When the speed differential computed by the speed differential computerfor the left and/or right steering clutch is more than a specificthreshold, the stall state determination component preferably determinesthe drive wheel corresponding to the steering clutch whose speeddifferential is more than the threshold to be in a stall state.

The clutch capacity of the steering clutch when the second pressure isexerted on the steering clutch is preferably equal to or greater thanthe maximum drive torque outputted from the power transmission device.

The steering clutch is preferably engaged when the clutch pressure ofthe steering clutch is the first pressure.

The steering clutch is preferably a hydraulic clutch. Preferably, thehigher is the pressure of the hydraulic fluid supplied to the steeringclutch, the more the clutch pressure rises.

Effects of the Invention

(A) With the work vehicle discussed above, if the vehicle speed isdetermined to be equal to or greater than a specific speed, the workvehicle performs control to reduce the engagement pressure from thefirst pressure. That is, when the rotational speed of the rotary memberis high, the hydraulic pressure exerted on the sealing ring is reduced.Therefore, control is performed so that the PV value of the sealing ringdoes not reach the maximum permissible value.

(B) With the monitoring system according to the first mode, the workvehicle decides the pressure of the hydraulic fluid inside the hydraulicfluid supply channels, and outputs determination basis data related tothe decided pressure, the rotational speed, and the time controlled bythe decided pressure while the rotary member is rotating, in a formatthat can be inputted by the monitoring device. The monitoring deviceoutputs maintenance information about the sealing ring based on thepredicted wear amount of the sealing ring obtained from thedetermination basis data. Thus, a worker is properly notified aboutreplacement of the sealing ring and so forth according to the usagestate of the sealing ring.

With the work vehicle according to the second mode, the work vehicledecides the pressure of the hydraulic fluid inside the hydraulic fluidsupply channels, and outputs determination basis data related to thedecided pressure, the rotational speed, and the time controlled by thedecided pressure while the rotary member is rotating, in a format thatcan be inputted by the monitoring device. Therefore, the monitoringdevice can properly notify a worker about replacement of the sealingring and so forth according to the usage state of the sealing ring, onthe basis of the determination basis data.

With the work vehicle according to the third mode, the work vehicledecides the pressure of the hydraulic fluid inside the hydraulic fluidsupply channels, and outputs maintenance information about the sealingring based on the predicted wear amount of the sealing ring obtainedfrom the decided pressure, the rotational speed, and the time controlledby the decided pressure while the rotary member is rotating. Thus, thework vehicle can properly notify a worker about replacement of thesealing ring and so forth according to the usage state of the sealingring.

(C) With the tracked work vehicle discussed above, if it is determinedthat the drive wheels are in a stall state, the steering clutch pressureis raised to a second pressure that is higher than the first pressure,which is the normal engagement pressure. Therefore, the clutch is lesslikely to slip when a high load is exerted on the crawler belt. As aresult, the work can be carried out normally even when the crawler beltsare subjected to a high load, which helps avoid a decrease in workefficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an oblique view of a bulldozer in an embodiment;

FIG. 2 shows the simplified configuration of the bulldozer shown in FIG.1;

FIG. 3 is a detail view of the area around the left steering brake andthe left steering clutch;

FIG. 4 is a block diagram of the controller according to the firstembodiment;

FIG. 5 is a flowchart of a method for controlling a steering clutch;

FIG. 6 is a flowchart of a method for determining vehicle speed in thefirst embodiment;

FIG. 7 is an example of a method for changing clutch pressure in thefirst embodiment;

FIG. 8 is another example of a method for changing clutch pressure inthe first embodiment;

FIG. 9 is a block diagram of the controller according to a secondembodiment;

FIG. 10 is an example of a graph of bulldozer travel performance;

FIG. 11 is a flowchart of a method for determining vehicle speed in thesecond embodiment;

FIG. 12 is an example of a method for changing the clutch pressure inthe second embodiment;

FIG. 13 is an oblique view of a bulldozer in an embodiment;

FIG. 14 shows the simplified configuration of the bulldozer shown inFIG. 13;

FIG. 15 is a detail view of the area around the left steering brake andthe left steering clutch;

FIG. 16 is a block diagram of the controller according to the firstembodiment;

FIG. 17 is a flowchart of a method for monitoring the bulldozer in thefirst embodiment;

FIG. 18 is a diagram of the overall configuration of the monitoringsystem according to the second embodiment;

FIG. 19 is a flowchart of the bulldozer operation in the monitoringmethod used in the monitoring system according to the second embodiment;

FIG. 20 is a flowchart of the monitoring device operation in themonitoring method used in the monitoring system according to the secondembodiment;

FIG. 21 is a block diagram of the controller according to a thirdembodiment;

FIG. 22 is a flowchart of the detailed operation of step 1 in abulldozer according to the third embodiment;

FIG. 23 is an example of a method for changing the clutch pressure inthe third embodiment;

FIG. 24 is an another example of a method for changing clutch pressurein the third embodiment;

FIG. 25 is an oblique view of a bulldozer in an embodiment;

FIG. 26 shows the simplified configuration of the bulldozer shown inFIG. 25;

FIG. 27 is a block diagram of the bulldozer according to the firstembodiment;

FIG. 28 is a flowchart of a method for controlling a steering clutch;

FIG. 29 is a flowchart of a method for determining a stall stateaccording to the first embodiment;

FIG. 30 is a block diagram of the controller according to the secondembodiment; and

FIG. 31 is a flowchart of a method for determining a stall stateaccording to the second embodiment.

DETAILED DESCRIPTION OF EMBODIMENT(S)

(A)

First Embodiment

FIG. 1 shows a bulldozer 1, which is an example of a work vehicle. Asshown in FIGS. 1 and 2, the bulldozer 1 includes left and right driveunits 4L and 4R respectively having sprockets 2L and 2R and crawlerbelts 3L and 3R, a blade 5 provided at the front of the vehicle, and aripper device 6 provided at the rear of the vehicle. This bulldozer 1can perform work such as dozing with the blade 5, or work such ascrushing or excavation with the ripper device 6.

The bulldozer 1 further includes a cab 7 above the left and right driveunits 4L and 4R. The cab 7 is equipped with a seat in which the operatorsits, various kinds of control lever, a vehicle speed setting switch,pedals, gauges, and so forth. In the following description, the “forwardand backward direction” means the forward and backward direction of thebulldozer 1. The forward and backward direction means the forward andbackward direction as seen by the operator seated in the cab 7. The leftand right direction or “to the side” refers to the vehicle widthdirection of the bulldozer 1. The left and right direction, the vehiclewidth direction, and “to the side” all refer to the left and rightdirections as seen by the above-mentioned operator.

Configuration of Power Transmission System

As shown in FIG. 2, this bulldozer 1 includes an engine 10 and a powertransmission device 11 that transmits power from the engine 10 to theleft and right drive units 4L and 4R. The power transmission device 11includes left and right steering clutches 12L and 12R, left and rightsteering brakes 13L and 13R, a torque converter 16, and a transmission17.

The power from the engine 10 is transmitted to a power takeoff 15. Thepower takeoff 15 sends part of the power from the engine 10 to hydraulicpumps or the like that generate power for the blade 5 and the ripperdevice 6, and sends the rest of the power to the torque converter 16.The torque converter 16 transmits power through a fluid. The outputshaft of the torque converter 16 is linked to the input shaft of thetransmission 17, and power is transmitted from the torque converter 16to the transmission 17.

The transmission 17 changes the speed of the rotary motion of theengine. The transmission 17 is provided with a clutch 17 a for switchingbetween forward and reverse, and a plurality of shifting clutches 17 b.The clutches 17 a and 17 b are hydraulic clutches that can behydraulically switched between engaged and disengaged states. The supplyand discharge of hydraulic fluid to and from the clutches 17 a and 17 bare controlled by a transmission control valve 26. The power outputtedfrom the transmission 17 is transmitted through a first bevel gear 18and a second bevel gear 19 to a lateral shaft 20.

The power transmitted to the lateral shaft 20 goes through the leftsteering clutch 12L, a left output shaft 21L, and a left final reductiongear 22L, and is transmitted to the left sprocket 2L. Also, the powertransmitted to the lateral shaft 20 goes through the right steeringclutch 12R, a right output shaft 21R, and a right final reduction gear22R, and is transmitted to the right sprocket 2R. The crawler belts 3Land 3R are wound around the sprockets 2L and 2R. Therefore, when thesprockets are rotationally driven, the crawler belts 3L and 3R aredriven, and this propels the bulldozer 1.

The bulldozer 1 includes left and right rotational speed sensors 42L and42R that sense the rotational speed of the left and right output shafts21L and 21R, for the purpose of sensing the vehicle speed of thebulldozer 1, etc. In the following description, for the sake ofconvenience, the sprockets 2L and 2R may also be called drive wheels.

The left and right steering clutches 12L and 12R are respectivelydisposed between the transmission 17 and the sprockets 2L and 2R, andare hydraulic clutches that can be hydraulically switched betweenengaged and disengaged states. The supply and discharge of hydraulicfluid to and from the steering clutches 12L and 12R are controlled bysteering clutch pressure control valves 27L and 27R.

Here, if the left steering clutch 12L is in its engaged state, powerfrom the second bevel gear 19 is transmitted to the left sprocket 2L. Ifthe left steering clutch 12L is in its disengaged state, power from thesecond bevel gear 19 is cut off by the left steering clutch 12L, and isnot transmitted to the left sprocket 2L. If the right steering clutch12R is in its engaged state, power from the second bevel gear 19 istransmitted to the tight sprocket 2R. If the right steering clutch 12Ris in its disengaged state, power from the second bevel gear 19 is cutoff by the right steering clutch 12R, and is not transmitted to theright sprocket 2R.

The left and right steering brakes 13L and 13R are respectively disposedbetween the left and right steering clutches 12L and 12R and thesprockets 2L and 2R, and are hydraulic brakes that can be hydraulicallyswitched between a braking state and a non-braking state. The supply anddischarge of hydraulic fluid to and from the left and right steeringbrakes 13L and 13R are controlled by brake pressure control valves 28Land 28R.

The output rotation of the left steering clutch 12L, that is, therotation of the left sprocket 2L, can be braked by putting the leftsteering brake 13L in a braking state. The output rotation of the rightsteering clutch 12R, that is, the rotation of the right sprocket 2R, canbe braked by putting the right steering brake 13R in a braking state.

With the above configuration, in a state in which the left steeringclutch 12L is disengaged and the left steering brake 13L is braking, ifthe right steering clutch 12R is engaged and the right sprocket 2R isrotationally driven, the bulldozer 1 will turn to the left. Conversely,in a state in which the right steering clutch 12R is disengaged and theright steering brake 13R is braking, if the left steering clutch 12L isengaged and the left sprocket 2L is rotationally driven, the bulldozer 1will turn to the right.

Configuration Around Steering Clutches

Referring to FIG. 3, a first left rotary member 56L is linked to oneside of the left steering clutch 12L, and a second left rotary member57L is linked to the other side. That is, the power transmission device11 includes the first left rotary member 56L and the second left rotarymember 57L.

The first left rotary member 56L is engaged by spline mating with thelateral shaft 20. The first left rotary member 56L rotates integrallywith the second bevel gear 19 and the lateral shaft 20. The second leftrotary member 57L is engaged by spline mating with the left output shaft21L linked to the left final reduction gear 22L. Therefore, the leftdrive unit 4L is driven by the second left rotary member 57L. The secondleft rotary member 57L is rotatably supported by a first left bearing66L disposed on the first left rotary member 56L. The power transmissiondevice 11 includes the first left bearing 66L.

A first left support member 58L is linked to one side of the leftsteering brake 13L, and the second left rotary member 57L is linked tothe other side. The first left support member 58L is fixed to a housing50 of the power transmission device 11. The first left support member58L and the housing 50 are collectively referred to here as a supportmember. The power transmission device 11 includes the support member.

A second left bearing 67L and a third left bearing 68L are attached onthe support member. The power transmission device 11 includes the secondleft bearing 67L and the third left bearing 68L. The second left bearing67L and the third left bearing 68L rotatably support the second leftrotary member 57L and the left output shaft 21L. Consequently, thesupport member rotatably supports the second left rotary member 57L andthe left output shaft 21L. The second left rotary member 57L and theleft output shaft 21L rotate integrally.

The bulldozer 1 includes a hydraulic fluid supply channel 60 thatsupplies hydraulic fluid to the power transmission device 11, and morespecifically to the left and right steering clutch 12L and 12R and theleft and right steering brakes 13L and 13R inside the support member andthe second left rotary member 57L. The hydraulic fluid supply channel 60includes a first left hydraulic fluid supply channel 61L, a second lefthydraulic fluid supply channel 62L, and a left connected part 63L. Thefirst left hydraulic fluid supply channel 61L is formed inside thesecond left rotary member 57L, and connects to the left steering clutch12L. The first left hydraulic fluid supply channel 61L supplieshydraulic fluid to the left steering clutch 12L.

The second left hydraulic fluid supply channel 62L is formed inside thesupport member, that is, on the outside of the second left rotary member57L. The second left hydraulic fluid supply channel 62L supplieshydraulic fluid to the first left hydraulic fluid supply channel 61L.The left connected part 63L connects the first left hydraulic fluidsupply channel 61L to the second left hydraulic fluid supply channel62L. Also, the hydraulic fluid supply channel 60 separately includes abraking left supply channel 69L that connects to the left steering brake13L. In FIG. 3, the braking left supply channel 69L is disposedoverlapping the first left hydraulic fluid supply channel 61L, and thebraking left supply channel 69L is located on the back side of the firstleft hydraulic fluid supply channel 61L.

The left connected part 63L is provided in a gap between the supportmember and the second left rotary member 57L. The power transmissiondevice 11 includes a first left sealing ring 64L and a second leftsealing ring 65L. The first left sealing ring 64L and the second leftsealing ring 65L are disposed adjacent to the left connected part 63L sothat hydraulic fluid will not leak out from the left connected part 63L.More specifically, the first left sealing ring 64L is mounted on thesurface of the second left rotary member 57L on the side closer to thesecond bevel gear 19 than the left connected part 63L (on the inside ofthe vehicle). The second left sealing ring 65L is mounted on the surfaceof the second left rotary member 57L on the side closer to the leftfinal reduction gear 22L than the left connected part 63L (on theoutside of the vehicle).

The gap between the support member and the second left rotary member 57Lis sealed by the first left sealing ring 64L and the second left sealingring 65L. Therefore, the left connected part 63L is formed by the gapbetween the support member and the second left rotary member 57L, whichis sealed by the first left sealing ring 64L and the second left sealingring 65L. In the example in FIG. 3, the shaft diameters of the firstleft sealing ring 64L and the second left sealing ring 65L are equal,but this is not necessarily the case.

FIG. 3 shows only the first left rotary member 56L, the second leftrotary member 57L, the first left support member 58L, the first lefthydraulic fluid supply channel 61L, the second left hydraulic fluidsupply channel 62L, the left connected part 63L, the braking left supplychannel 69L, the first left sealing ring 64L, the second left sealingring 65L, the first left bearing 66L, the second left bearing 67L, andthe third left bearing 68L.

However, the hydraulic fluid supply channel 60 also includes a firstright hydraulic fluid supply channel 61R, a second right hydraulic fluidsupply channel 62R, a right connected part 63R, and a braking rightsupply channel 69R, which are connected to the right steering clutch 12Ror the right steering brake 13R. The first right hydraulic fluid supplychannel 61R, the second right hydraulic fluid supply channel 62R, theright connected part 63R, and the braking right supply channel 69Rrespectively have the same structure as the first left hydraulic fluidsupply channel 61L, the second left hydraulic fluid supply channel 62L,the left connected part 63L, and the braking left supply channel 69L.

Also, the power transmission device 11 includes a first right rotarymember 56R, a second right rotary member 57R, and a first right supportmember 58R, which are connected to the left steering clutch 12R or theright steering brake 13R. The first right rotary member 56R, the secondright rotary member 57R, and the first right support member 58R have thesame structure as the first left rotary member 56L, the second leftrotary member 57L, and the first left support member 58L.

The power transmission device 11 includes a first right sealing ring64R, a second right sealing ring 65R, a first right bearing 66R, asecond right bearing 67R, and a third right bearing 68R, which areconnected to the first right rotary member 56R, the second right rotarymember 57R, or the first right support member 58R. The first rightsealing ring 64R, the second right sealing ring 65R, the first rightbearing 66R, the second right bearing 67R, and the third right bearing68R respectively have the same structure as the first left sealing ring64L, the second left sealing ring 65L, the first left bearing 66L, thesecond left bearing 67L, and the third left bearing 68L.

The left steering clutch 12L is a wet multi-plate type, and includesclutch disks 51 and a clutch piston 52. With the left steering clutch12L, when the hydraulic pressure of the hydraulic fluid from the firstleft hydraulic fluid supply channel 61L is applied to the clutch piston52, the clutch disks 51 are joined by hydraulic pressure and power istransmitted. Therefore, the left steering clutch 12L is engaged whensubjected to the pressure of the hydraulic fluid supplied from thehydraulic fluid supply channel 60 to the left steering clutch 12L.

Saying that the left steering clutch 12L is engaged means that apressure equal to or greater than a specific holding pressure, which isthe pressure at which torque within the designed range can betransmitted without clutch slippage, is being supplied to the leftsteering clutch 12L. Here, the pressure equal to or greater than aspecific holding pressure that is applied to the steering clutches 12Land 12R in the engagement of the steering clutches 12L and 12R is calledthe engagement pressure. Normally, when the left steering clutch 12L isengaged, the pressure of the hydraulic fluid applied to the leftsteering clutch 12L is a first pressure. This first pressure is includedin the engagement pressure.

The pressure applied to the left steering clutch 12L (clutch pressure)rises in proportion to the hydraulic pressure supplied to the leftsteering clutch 12L. In FIG. 3, only the left steering clutch 12L isshown, but the right steering clutch 12R has the same structure.

The left steering brake 13L is a wet multi-plate type, and includesbrake disks 53, a brake piston 54, and a plate spring 55. The leftsteering brake 13L is a so-called negative brake, with which the brakedisks 53 are pressed and a braking state is produced by the biasingforce of the plate spring 55 in a state in which the hydraulic pressureof the hydraulic fluid from the braking left supply channel 69L is notbeing applied to the brake piston 54.

When the hydraulic pressure of the hydraulic fluid from the braking leftsupply channel 69L is applied to the brake piston 54, the brake piston54 causes the brake disks 53 to move apart against the biasing force ofthe plate spring. This puts the left steering brake 13L in a releasedstate. In FIG. 3, only the left steering brake 13L is shown, but theright steering brake 13R has the same structure.

Here, when braking is produced by the left steering brake 13L, the leftsteering clutch 12L is released. At this point the second left rotarymember 57L is stationary along with the support member. Therefore, therotational speed of the first left sealing ring 64L and the second leftsealing ring 65L is zero, and there is little hydraulic pressure appliedto the first left sealing ring 64L and the second left sealing ring 65L.Therefore, the PV value of the first left sealing ring 64L and thesecond left sealing ring 65L falls well within the permissible usagerange. The PV value is the product of the surface pressure P applied tothe rotary member and the slipping velocity V. The same can be the ofthe right steering brake 13R.

Meanwhile, when the left steering brake 13L is released and the leftsteering clutch 12L is engaged, the second left rotary member 57Lrotates along with the first left rotary member 56L under the power fromthe transmission 17. The support member, however, is stationary.Therefore, the slipping velocities V of the first left sealing ring 64Land the second left sealing ring 65L respectively correspond to theproduct of multiplying the shaft diameters of the first left sealingring 64L and the second left sealing ring 65L by the circumference ratioand the rotational speed of the second left rotary member 57L.

When no slip is occurring at the left steering clutch 12L, therotational speed of the second left rotary member 57L is equal to therotational speed of the first left rotary member 56L. Furthermore, sincethe left steering clutch 12L is engaged, the surface pressure P appliedto the first left sealing ring 64L and the second left sealing ring 65Lis normally the above-mentioned first pressure.

When the rotational speed of the second left rotary member 57L rises, orwhen the clutch pressure of the left steering clutch 12L increases,there is an increase in the PV values of the first left sealing ring 64Land the second left sealing ring 65L. Therefore, the PV values of thefirst left sealing ring 64L and the second left sealing ring 65L mustfall within the permissible usage range (a range which is equal to orless than the maximum permissible value). The same can be the of theright steering clutch 12R.

Configuration for Output Control

This bulldozer 1 has a controller 30 (see FIG. 2). The controller 30includes a CPU or other such computation device, and a RAM, ROM, orother such memory device. The controller 30 is connected to a fuel dial45, a decelerator pedal 46, a steering lever 48, and anupshift/downshift button 49, which are housed in the cab 7.

The fuel dial 45 is operated by the operator. A signal indicating thetarget engine speed, which is the amount the fuel dial 45 is turned, isinputted to the controller 30. The decelerator pedal 46 is depressed bythe operator. A signal indicating the deceleration rotational speed,which is the amount the decelerator pedal 46 is depressed, is inputtedto the controller 30. The controller 30 sends an engine output commandto the engine 10 and controls the engine 10 so as to obtain a targetengine speed corresponding to how much the fuel dial 45 is turned. Thecontroller 30 also controls the speed of the engine 10 so that theengine speed will decrease according to how much the decelerator pedal46 is depressed.

The steering lever 48 is used to switch the bulldozer 1 between forwardand reverse movement and to switch its turning direction. Theupshift/downshift button 49 is used by the operator to shift the gear ofthe transmission 17. The controller 30 receives a signal from thesteering lever 48 or the upshift/downshift button 49 and shifts the gearof the transmission 17 and controls the pressure control valves 27L,27R, 28L, and 28R.

FIG. 4 is a block diagram of the detailed configuration of thecontroller 30 according to the first embodiment. The controller 30includes a vehicle speed determination component 31 and a pressurecontroller 32. Typically, programs and data for executing the variousfunctions of the vehicle speed determination component 31 and thepressure controller 32 are stored in the memory device. When thecomputation device executes the program, the controller 30 executes thevarious functions of the vehicle speed determination component 31 andthe pressure controller 32. The controller 30 may also be realized by anintegrated circuit.

The vehicle speed determination component 31 determines whether or notthe vehicle speed of the bulldozer 1 is equal to or greater than aspecific speed. The rotational speeds of the left and right rotationalspeed sensors 42L and 42R (see FIG. 2) are inputted to the vehicle speeddetermination component 31, for example. The rotational speed measuredby the left rotational speed sensor 42L corresponds to the rotationalspeed of the first left sealing ring 64L and the second left sealingring 65L. The rotational speed measured by the right rotational speedsensor 42R corresponds to the rotational speed of the first rightsealing ring 64R and the second right sealing ring 65R.

More specifically, when the rotational speed ΦL measured by the leftrotational speed sensor 42L is greater than a specific rotational speedthreshold ΦthL, or when rotational speed ΦR measured by the rightrotational speed sensor 42R is greater than a specific rotational speedthreshold ΦthR, the vehicle speed determination component 31 determinesthe vehicle speed of the bulldozer 1 to be equal to or greater than thespecific speed. The rotational speed threshold ΦthL is predetermined soas to satisfy the following formula (1). The rotational speed thresholdΦthR is predetermined so as to satisfy the following formula (2).

ΦthL≦MaxPVL/(DL×P1)  (1)

MaxPVL: maximum permissible PV value of first left sealing ring 64L andsecond left sealing ring 65LDL: shaft diameter of first left sealing ring 64L and second leftsealing ring 65LP1: first pressure

ΦthR≦MaxPVR/(DR×P1)  (2)

MaxPVR: maximum permissible PV value of first right sealing ring 64R andsecond right sealing ring 65RDR: shaft diameter of first right sealing ring 64R and second rightsealing ring 65RP1: first pressure

When the first left sealing ring 64L, the second left sealing ring 65L,the first right sealing ring 64R, and the second right sealing ring 65Rare all the same sealing ring, the right side of Formula 1 and the rightside of Formula 2 are equivalent. In this case, it is preferable ifΦthL=ΦthR.

In the above description, an example is given of a case in which themaximum permissible PV values of the first left sealing ring 64L and thesecond left sealing ring 65L are the same as the maximum permissible PVvalues of the first right sealing ring 64R and the second right sealingring 65R, and the shaft diameters of the first left sealing ring 64L andthe second left sealing ring 65L are the same as the shaft diameters ofthe first right sealing ring 64R and the second right sealing ring 65R.

However, there are also cases when the first left sealing ring 64L andthe second left sealing ring 65L have different maximum permissible PVvalues. Or, the first left sealing ring 64L and the second left sealingring 65L may have different shaft diameters. In such a case,MaxPVL/(DL×P1) may be calculated for each of the first left sealing ring64L and the second left sealing ring 65L, and the rotational speedthreshold ΦthL set for the first left sealing ring 64L and the secondleft sealing ring 65L so as to be equal to or less than the lower of thecalculated values.

Similarly, there are also cases when the first right sealing ring 64Rand the second right sealing ring 65R have different maximum permissiblePV values. Or, the first right sealing ring 64R and the second leftsealing ring second right sealing ring 65R may have different shaftdiameters. In such a case, MaxPVR/(DR×P1) may be calculated for each ofthe first right sealing ring 64R and the second right sealing ring 65R,and the rotational speed threshold ΦthR set for the first right sealingring 64R and the second right sealing ring 65R so as to be equal to orless than the lower of the calculated values.

The pressure controller 32 decides the pressure of the hydraulic fluidin the hydraulic fluid supply channel 60 so as to keep the PV value ofthe left and right steering clutches 12L and 12R within the permissibleusage range. More specifically, the pressure controller 32 decides theengagement pressure of the left steering clutch 12L so as to keep the PVvalue of the left steering clutch 12L within the permissible usagerange. The pressure controller 32 then outputs a command correspondingto the decided engagement pressure to the left steering clutch pressurecontrol valve 27L. The pressure controller 32 performs control so thatthe clutch pressure of the left steering clutch 12L will be the decidedengagement pressure.

Similarly, the pressure controller 32 decides the engagement pressure ofthe right steering clutch 12R so as to keep the PV value of the rightsteering clutch 12R within the permissible usage range. The pressurecontroller 32 then outputs a command corresponding to the decidedengagement pressure to the right steering clutch pressure control valve27R. The pressure controller 32 performs control so that the clutchpressure of the right steering clutch 12R will be the decided engagementpressure. The steering clutch pressure control valves 27L and 27Rreceive commands from the pressure controller 32, and control thepressure of the hydraulic fluid in the hydraulic fluid supply channel60.

More specifically, if the vehicle speed determination component 31determines the vehicle speed of the bulldozer 1 not to be equal to orgreater than a specific speed, the pressure controller 32 outputs acommand to the steering clutch pressure control valves 27L and 27R touse the above-mentioned first pressure for the pressure of the hydraulicfluid in the hydraulic fluid supply channel 60. That is, the pressurecontroller 32 performs control so that the engagement pressure of theleft and right steering clutches 12L and 12R will be the first pressure.

If the vehicle speed determination component 31 determines the vehiclespeed of the bulldozer 1 to be equal to or greater than the specificspeed, the pressure controller 32 outputs a command to the steeringclutch pressure control valves 27L and 27R to reduce the pressure of thehydraulic fluid in the hydraulic fluid supply channel 60 from the firstpressure. That is, the pressure controller 32 performs control so thatthe engagement pressure of the left and right steering clutches 12L and12R will be reduced from the first pressure.

More specifically, if the vehicle speed determination component 31determines the vehicle speed of the bulldozer 1 to be equal to orgreater than the specific speed, the pressure controller 32 outputs acommand to the left steering clutch pressure control valve 27L to reducethe hydraulic pressure PL so that the product of the rotational speed ΦLand the hydraulic pressure PL applied to the left steering clutch 12Lwill not exceed a specific upper limit UlimL. That is, the pressurecontroller 32 performs control so that the pressure will be reduced fromthe engagement pressure PL so that the product of the rotational speedΦL and the engagement pressure PL of the left steering clutch 12L willnot exceed the specific upper limit UlimL.

Similarly, if the vehicle speed determination component 31 determinesthe vehicle speed of the bulldozer 1 to be equal to or greater than thespecific speed, the pressure controller 32 outputs a command to theright steering clutch pressure control valve 27R to reduce the hydraulicpressure PR so that the product of the rotational speed ΦR and thehydraulic pressure PR applied to the right steering clutch 12R will notexceed a specific upper limit UlimR. That is, the pressure controller 32performs control so that the pressure will be reduced from theengagement pressure PR so that the product of the rotational speed ΦRand the engagement pressure PR of the right steering clutch 12R will notexceed the specific upper limit UlimR. These specific upper limits UlimLand UlimR are determined by Formulas 3 and 4.

UlimL=ΦthL×P1  (3)

ΦthL: rotational speed threshold determined so as to satisfy Formula 1P1: first pressure

UlimR=ΦthR×P1  (4)

ΦthR: rotational speed threshold determined so as to satisfy Formula 2P1: first pressure

These upper limits UlimL and UlimR may be called a first product. Thepressure controller 32 can make use of the first product UlimL to setthe target hydraulic pressure PL applied to the left steering clutch 12Las in Formula 5. Similarly, the pressure controller 32 can make use ofthe first product UlimR to set the target hydraulic pressure PR appliedto the right steering clutch 12R as in Formula 6. The pressurecontroller 32 outputs commands to the steering clutch pressure controlvalves 27L and 27R to use the target hydraulic pressures PL and PR thusset. The pressure controller 32 controls the engagement pressure of theleft and right steering clutches 12L and 12R to be the target hydraulicpressures PL and PR thus set.

PL=UlimL/MAX(ΦL,ΦR)  (5)

MAX (ΦL, ΦR): rotational speed of the greater of ΦL and ΦR

PR=UlimR/MAX(ΦL,ΦR)  (6)

MAX (ΦL, ΦR): rotational speed of the greater of ΦL and ΦR

If the rotational speed ΦL of the left rotational speed sensor 42L ishigher than the rotational speed threshold ΦthL, the pressure controller32 may control the engagement pressure PL of the left steering clutch12L regardless of the rotational speed ΦR of the right rotational speedsensor 42R. Similarly, if the rotational speed ΦR of the rightrotational speed sensor 42R is higher than the rotational speedthreshold ΦthR, the pressure controller 32 may control the engagementpressure PR of the right steering clutch 12R regardless of therotational speed ΦL of the left rotational speed sensor 42L.

In this case, the pressure controller 32 preferably outputs a command tothe pressure control valve 27L of the left steering clutch 12L to usethe pressure PL found from Formula 7 for the engagement pressure of theleft steering clutch 12L. Similarly, the pressure controller 32preferably outputs a command to the pressure control valve 27R of theright steering clutch 12R to use the pressure PR found from Formula 8for the engagement pressure of the right steering clutch 12R.

$\begin{matrix}{{PL} = \left\{ \begin{matrix}{P\; 1} & \left( {{VL} \leq {{VthL}}} \right) \\{U\mspace{11mu} \lim \mspace{11mu} {L/{VL}}} & \left( {{VL} > {{VthL}}} \right)\end{matrix} \right.} & (7) \\{{PR} = \left\{ \begin{matrix}{P\; 1} & \left( {{VR} \leq {{VthR}\mspace{11mu} }} \right) \\{U\mspace{11mu} \lim \mspace{11mu} {R/{VR}}} & \left( {{VR} > {{VthR}\mspace{11mu} }} \right)\end{matrix} \right.} & (8)\end{matrix}$

Next, the method for controlling the steering clutches 12L and 12R willbe described using a flowchart. FIG. 5 is a flowchart of the method forcontrolling the steering clutches. FIG. 6 is a flowchart of the vehiclespeed determination method according to the first embodiment. FIG. 7 isan example of a method for changing the clutch pressure in the firstembodiment. FIG. 8 is another example of a method for changing theclutch pressure in the first embodiment.

First, in step 1, the vehicle speed determination component 31determines whether or not the vehicle speed is equal to or greater thana specific speed. In this step, the left rotational speed sensor 42Lmeasures the rotational speed ΦL of the first left sealing ring 64L andthe second left sealing ring 65L in step 10 in FIG. 6, and the rightrotational speed sensor 42R measures the rotational speed ΦR of thefirst right sealing ring 64R and the second right sealing ring 65R. Instep 11, the vehicle speed determination component 31 determines whetheror not the rotational speed ΦL measured by the left rotational speedsensor 42L is more than a specific rotational speed threshold ΦthL.

If the rotational speed ΦL is not more than the rotational speedthreshold ΦthL (No in step 11), in step 12 the vehicle speeddetermination component 31 determines whether or not the rotationalspeed ΦR measured by the right rotational speed sensor 42R is more thana specific rotational speed threshold ΦthR.

If the rotational speed ΦL is more than the rotational speed thresholdΦthL (Yes in step 11), or if the rotational speed ΦR is more than thespecific rotational speed threshold ΦthR (Yes in step 12), in step 13the vehicle speed determination component 31 determines the vehiclespeed to be equal to or greater than the specific speed. If therotational speed ΦR is not more than the rotational speed threshold ΦthR(No in step 12), in step 14 the vehicle speed determination component 31determines the vehicle speed not to be equal to or greater than thespecific speed.

If the vehicle speed determination component 31 determines the vehiclespeed to be equal to or greater than the specific speed (Yes in step 1in FIG. 5), in step 2 the pressure controller 32 reduces the engagementpressure (the pressure in the first left/right hydraulic fluid supplychannels) from the above-mentioned first pressure. More specifically,when the rotational speed ΦL is more than the specific rotational speedthreshold ΦthL, the pressure controller 32 reduces the engagementpressure PL so that the product of the rotational speed ΦL and theengagement pressure PL of the left steering clutch 12L will not exceedthe upper limit UlimL. Also, when the rotational speed ΦR is more thanthe specific rotational speed threshold ΦthR, the pressure controller 32reduces the engagement pressure PR so that the product of the rotationalspeed ΦR and the engagement pressure PR of the right steering clutch 12Rwill not exceed the upper limit UlimR.

An example of specific processing is shown in FIG. 7. In step 21, thepressure controller 32 performs control so that the engagement pressurePL of the left steering clutch 12L will be UlimL/MAX (ΦL, ΦR), and theengagement pressure PR of the right steering clutch 12R will beUlimR/MAX (ΦL, ΦR).

Another processing example is shown in FIG. 8. In step 22, when therotational speed ΦL is equal to or less than the rotational speedthreshold ΦthL, the pressure controller 32 performs control so that theengagement pressure PL will be the first pressure P1. When therotational speed ΦL is more than the rotational speed threshold ΦthL,the pressure controller 32 performs control so that the engagementpressure PL will be UlimL/ΦL. When the rotational speed ΦR is equal toor less than the rotational speed threshold ΦthR, the pressurecontroller 32 performs control so that the engagement pressure PR willbe the first pressure P1. When the rotational speed ΦR is more than therotational speed threshold ΦthR, the pressure controller 32 performscontrol so that the engagement pressure PR will be UlimR/ΦR.

If the vehicle speed determination component 31 determines that thevehicle speed is not equal to or greater than a specific speed (No instep 1 in FIG. 5), in step 3 the pressure controller 32 performs controlso that the engagement pressure (the pressure in first left/righthydraulic fluid supply channels) will be the above-mentioned firstpressure.

Second Embodiment

The method for determining whether or not the vehicle speed is equal toor greater than the specific speed, and the method for changing theengagement pressure are not limited to the methods in the aboveembodiment, and there are other methods. In this second embodiment acontroller 30 a that is different from the controller 30 in the firstembodiment will be described. In the second embodiment, the componentsother than the controller 30 a are all the same as in the firstembodiment, and will therefore not be described again. Also, thecontroller 30 a is configured the same as the controller 30 in the firstembodiment, except for a vehicle speed determination component 31 a anda pressure controller 32 a (discussed below). Therefore, descriptionrelated to components that are the same as in the controller 30 will beomitted.

FIG. 9 is a block diagram of the detailed configuration of thecontroller 30 a according to the second embodiment. The controller 30 aincludes the vehicle speed determination component 31 a and the pressurecontroller 32 a. The vehicle speed determination component 31 adetermines whether or not the vehicle speed of the bulldozer 1 is equalto or greater than the specific speed. The gear sensed by a gear sensor47 is also inputted to the vehicle speed determination component 31 a.The gear sensor 47 is connected to the upshift/downshift button 49 andsenses the gear of the power transmission device 11 set by operatorinput. The controller 30 a may automatically shift the gear of thetransmission 17. In this case, the gear sensor 47 preferably senses thegear to which the transmission 17 has been automatically shifted.

More specifically, when the gear sensed by the gear sensor 47 is eithera first specific gear or is a gear higher than the first gear, thevehicle speed determination component 31 a determines the vehicle speedof the bulldozer 1 to be equal to or greater than the specific speed.The reason for this will be explained through reference to FIG. 10. FIG.10 is an example of a graph of the travel performance in the variousgears of the bulldozer 1. GS1, GS2, and GS3 in FIG. 10 indicate thetractive power in first, second, and third gears, respectively.

The tractive power in each gear goes up or down depending on a number ofconditions, such as (1) the position of the fuel dial 45 and (2) howmuch the decelerator pedal 46 is depressed, but these conditions are thesame at GS1, GS2, and GS3. In FIG. 10, if the tractive power isnegative, that means that a force is being applied to slow down thevehicle (known as engine braking).

FIG. 10 shows an example of when the bulldozer 1 has three gears. Ifthere are four or more gears, as the gear rises to fourth and fifth, thetravel performance curve for each gear varies such that the tractivepower at a speed of 0 decreases, and the speed at a tractive power of 0increases. The number of gears of the bulldozer 1 is not limited to whatis shown in FIG. 10, and may be two gears, or four or more gears.

The first specific gear here is generally the highest gear that can beset on the bulldozer 1. That is, in the example in FIG. 10, the firstspecific gear means third gear. When the bulldozer 1 travels underautomatic gear shifts, the gear is generally shifted in order to obtainthe tractive power needed for the load. The shift speed for that gear isset near the intersection of the traction curve. For instance, the shiftspeed from third gear to second is a vehicle speed v2. Therefore, in thecase in FIG. 10, the fact that the gear is third means that there is ahigh probability that the vehicle speed is equal to or greater than v2.

In this case, the rotational speed ΦL of the sealing rings 64L and 65Land the rotational speed ΦR of the sealing rings 64R and 65R are higher,and there is the possibility that ΦL and ΦR will exceed the rotationalspeed thresholds ΦthL and ΦthR set to the ranges defined by Formulas 1and 2, respectively. Therefore, the pressure controller 32 a performscontrol so that when the vehicle speed determination component 31 a hasdetermined the vehicle speed of the bulldozer 1 to be equal to orgreater than the specific speed, the engagement pressure of the left andright steering clutches 12L and 12R will be reduced from the firstpressure.

However, the first gear may be set lower than the highest gear, and setto a gear near the highest gear. For example, in the example in FIG. 10,the first gear may be second gear. In this case, it means that there isa high possibility that the vehicle speed is equal to or greater thanv1. The “first gear” is never set to actual first.

In FIG. 9, if the vehicle speed determination component 31 a determinesthe vehicle speed of the bulldozer 1 to be equal to or greater than thespecific speed, the pressure controller 32 a performs control so thatthe engagement pressure PL of the left steering clutch 12L will be asecond pressure that is lower than the above-mentioned first pressure.Similarly, if the vehicle speed determination component 31 a determinesthe vehicle speed of the bulldozer 1 to be equal to or greater than thespecific speed, the pressure controller 32 a performs control so thatthe engagement pressure PR of the right steering clutch 12R will be thesecond pressure. The second pressure is included in the engagementpressure.

If the designed maximum rotational speed of the left output shaft 21Land the right output shaft 21R when the gear is the first gear or a gearhigher than the first gear is set to Φmax, then the second pressure P2will be as shown in the following Formula 9.

P2≦Min(MaxPVL/(DL×Φmax), MaxPVR/(DR×Φmax))  (9)

MaxPVL: maximum permissible PV value of first left sealing ring 64L andsecond left sealing ring 65LMaxPVR: maximum permissible PV value of first right sealing ring 64R andsecond right sealing ring 65RDL: shaft diameter of first left sealing ring 64L and second leftsealing ring 65LDR: shaft diameter of first right sealing ring 64R and second rightsealing ring 65RMin (A, B): the lesser value of A and B (however, if the first leftsealing ring 64L, the second left sealing ring 65L, the first rightsealing ring 64R, and the second right sealing ring 65R are all the samesealing ring, then MaxPVL/(DL×Φmax)=MaxPVR/(DR×Φmax))

However, it is preferable for the second pressure P2 to be as high aspossible so that the left steering clutch 12L and the right steeringclutch 12R will be able to transmit as much torque as possible.

The method for controlling the steering clutches 12L and 12R will now bedescribed through reference to a flowchart. FIG. 11 is a flowchart of amethod for determining vehicle speed in the second embodiment. FIG. 12is an example of a method for changing the clutch pressure in the secondembodiment.

First, in step 15, the gear sensor 47 senses which gear the bulldozer 1is in. In step 16, the vehicle speed determination component 31 adetermines whether or not the gear is the first gear, or is a gearhigher than the first gear. If the gear is the first gear, or is a gearhigher than the first gear (Yes in step 16), in step 17 the vehiclespeed determination component 31 a determines the vehicle speed of thebulldozer 1 to be equal to or greater than the specific speed. If thegear is lower than the first gear (No in step 16), in step 18 thevehicle speed determination component 31 a determines the vehicle speedof the bulldozer 1 not to be equal to or greater than the specificspeed.

When the vehicle speed determination component 31 a determines thevehicle speed of the bulldozer 1 to be equal to or greater than thespecific speed (Yes in step 1 in FIG. 5), in step 23 in FIG. 12 thepressure controller 32 a performs control so that the engagementpressure PL of the left steering clutch 12L will be the above-mentionedsecond pressure. Similarly, the pressure controller 32 a performscontrol so that the engagement pressure PR of the right steering clutch12R will be the second pressure.

Features

(1) With the bulldozer 1 according to the first and second embodiments,if the vehicle speed is determined to be equal to or greater than thespecific speed, the pressure controllers 32 and 32 a perform control sothat the engagement pressure of the left and right steering clutches 12Land 12R will be reduced from the first pressure. That is, when therotational speed of the rotary member is high, the hydraulic pressureexerted on the sealing rings 64L, 65L, 64R, and 65R is reduced.Therefore, the PV value of the sealing rings 64L, 65L, 64R, and 65R iscontrolled so as not to reach the maximum permissible value.

(2) In the first embodiment, the vehicle speed determination component31 determines the vehicle speed to be equal to or greater than thespecific speed when the rotational speeds ΦL and ΦR measured by therotational speed sensors 42L and 42R are more than the specificrotational speed thresholds ΦthL and ΦthR. When the rotational speeds ΦLand ΦR are more than the specific rotational speed thresholds ΦthL andΦthR, the slipping velocity of the sealing rings 64L, 65L, 64R, and 65Ris high. Therefore, the PV value of the sealing rings 64L, 65L, 64R, and65R can be effectively controlled so as not to reach the maximumpermissible value.

(3) In the first embodiment, the pressure controller 32 performs controlso that when the rotational speeds ΦL and ΦR measured by the rotationalspeed sensors 42L and 42R are more than the rotational speed thresholdsΦthL and ΦthR, the engagement pressures PL and PR of the left and rightsteering clutches 12L and 12R are reduced so that the product of therotational speeds ΦL and ΦR and the pressures PL and PR of the hydraulicfluid will not exceed the specific upper limits UlimL and UlimR.

More specifically, the pressure controller 32 performs control so thatthe quotients of dividing the first products UlimL and UlimR, which arethe products of the first pressure P1 and the rotational speedthresholds ΦthL and ΦthR, by the rotational speeds ΦL and ΦR will be theengagement pressures PL and PR. Therefore, the PV value of the sealingrings 64L, 65L, 64R, and 65R can be reliably controlled so as not toreach the maximum permissible value.

(4) In the second embodiment, the vehicle speed determination component31 a determines the vehicle speed to be equal to or greater than thespecific speed when the gear is the first specific gear, or is higherthan the first specific gear. When the bulldozer 1 travels underautomatic gear shifts, the gear is generally shifted in order to obtainthe tractive power needed for the load. The shift speed for that gear isset near the intersection of the traction curve. This means that thereis a high probability that the vehicle speed will be equal to or greaterthan the lowest vehicle speed at which the tractive power in the firstspecific gear is the maximum. That is, it is highly probable that thevehicle speed will be equal to or greater than the specific speed.Therefore, the PV value of the sealing rings 64L, 65L, 64R, and 65R iscontrolled so as not to reach the maximum permissible value.

(5) In the second embodiment, the pressure controller 32 a performscontrol so that the engagement pressure become the second pressure,which is lower than the first pressure, when the gear is either thefirst gear or is higher than the first gear. As a result, when theslipping velocity of the sealing rings 64L, 65L, 64R, and 65R is high,the pressure applied to the sealing rings 64L, 65L, 64R, and 65Rdecreases. Therefore, the PV values of the sealing rings 64L, 65L, 64R,and 65R are effectively controlled so as not to reach the maximumpermissible value.

Modification Examples

The present invention is not limited to or by the above embodiments, andvarious modifications and variations are possible without departing fromthe scope of the present invention.

(a) In the above embodiments, an example was given in which two sealingrings were mounted per rotary member, but just one sealing ring, orthree or more, may be mounted per rotary member.

(b) In the above embodiments, a bulldozer was used as an example of awork vehicle, but the present invention can be similarly applied toother work vehicles in which a hydraulic clutch is provided to a powertransmission device.

(c) In the above embodiments, the first left hydraulic fluid supplychannel 61L may be formed in the first left rotary member 56L. In thiscase, the second left hydraulic fluid supply channel 62L is preferablyconnected to the first left rotary member 56L, and the first leftsealing ring 64L and the second left sealing ring 65L are mounted on thesurface of the first left rotary member 56L. Also, the first righthydraulic fluid supply channel 61R may be formed in the first rightrotary member 56R. In this case, the second right hydraulic fluid supplychannel 62R is preferably connected to the first right rotary member56R, and the first right sealing ring 64R and the second right sealingring 65R are mounted on the surface of the first right rotary member56R.

(d) Also, the first left hydraulic fluid supply channel 61L may be suchthat part of the supply channel is included in the left output shaft 21Lor in the lateral shaft 20, and the first right hydraulic fluid supplychannel 61R may be such that part of the supply channel is included inthe right output shaft 21R or in the lateral shaft 20. In this case, thesecond left hydraulic fluid supply channel 62L may be connected to theleft output shaft 21L or the lateral shaft 20, and the first leftsealing ring 64L and the second left sealing ring 65L may be mounted onthe surface of the left output shaft 21L or the lateral shaft 20. Thesecond right hydraulic fluid supply channel 62R may be connected to theright output shaft 21R or the lateral shaft 20, and the first rightsealing ring 64R and the second right sealing ring 65R may be mounted onthe surface of the right output shaft 21R or the lateral shaft 20.

(e) In the second embodiment, a plurality of first gears may be set.These set first gears shall be termed the set gear 1, the set gear 2, .. . , the set gear i, the set gear (i+1), . . . , and the set gear N,starting from the lowest gear. The set gear (i+1) (i=1, . . . , N−1) maybe one gear higher than the set gear i, or may be a number of gearshigher. Here, MAX (ΦL, ΦR) in Formulas 5 and 6 can be establishedaccording to the vehicle speed range from the gear corresponding to theset gear i (i=1, . . . , N−1) to a gear one lower than the gearcorresponding to the set gear (i+1) (the vehicle speed range of the gearwhen a shift is made so that the highest tractive power will beobtained). The values of the pressures PL and PR in Formulas 5 and 6 maythen be established as the second pressure.

Also, the second pressure does not necessarily have to be constant, andmay be suitably varied according to the elapsed time since the change tothe first gear. This is because as time passes since the change to thefirst gear, there is a high probability that the vehicle speed will haveincreased over the vehicle speed immediately after the change to thefirst gear.

(B)

First Embodiment

FIG. 13 shows a bulldozer 1, which is an example of a work vehicle. Asshown in FIGS. 13 and 14, the bulldozer 1 includes left and right driveunits 4L and 4R respectively having sprockets 2L and 2R and crawlerbelts 3L and 3R, a blade 5 provided at the front of the vehicle, and aripper device 6 provided at the rear of the vehicle. This bulldozer 1can perform work such as dozing with the blade 5, or work such ascrushing or excavation with the ripper device 6.

The bulldozer 1 further includes a cab 7 above the left and right driveunits 4L and 4R. The cab 7 is equipped with a seat in which the operatorsits, various kinds of control lever, a vehicle speed setting switch,pedals, gauges, and so forth. In the following description, the “forwardand backward direction” means the forward and backward direction of thebulldozer 1. The forward and backward direction means the forward andbackward direction as seen by the operator seated in the cab 7. The leftand right direction or “to the side” refers to the vehicle widthdirection of the bulldozer 1. The left and right direction, the vehiclewidth direction, and “to the side” all refer to the left and rightdirections as seen by the above-mentioned operator.

Configuration of Power Transmission System

As shown in FIG. 14, this bulldozer 1 includes an engine 10 and a powertransmission device 11 that transmits power from the engine 10 to theleft and right drive units 4L and 4R. The power transmission device 11includes left and right steering clutches 12L and 12R, left and rightsteering brakes 13L and 13R, a torque converter 16, and a transmission17.

The power from the engine 10 is transmitted to a power takeoff 15. Thepower takeoff 15 sends part of the power from the engine 10 to hydraulicpumps or the like that generate power for the blade 5 and the ripperdevice 6, and sends the rest of the power to the torque converter 16.The torque converter 16 transmits power through a fluid. The outputshaft of the torque converter 16 is linked to the input shaft of thetransmission 17, and power is transmitted from the torque converter 16to the transmission 17.

The transmission 17 changes the speed of the rotary motion of theengine. The transmission 17 is provided with a clutch 17 a for switchingbetween forward and reverse, and a plurality of shifting clutches 17 b.The clutches 17 a and 17 b are hydraulic clutches that can behydraulically switched between engaged and disengaged states. The supplyand discharge of hydraulic fluid to and from the clutches 17 a and 17 bare controlled by a transmission control valve 26. The power outputtedfrom the transmission 17 is transmitted through a first bevel gear 18and a second bevel gear 19 to a lateral shaft 20.

The power transmitted to the lateral shaft 20 goes through the leftsteering clutch 12L, a left output shaft 21L, and a left final reductiongear 22L, and is transmitted to the left sprocket 2L. Also, the powertransmitted to the lateral shaft 20 goes through the right steeringclutch 12R, a right output shaft 21R, and a right final reduction gear22R, and is transmitted to the right sprocket 2R. The crawler belts 3Land 3R are wound around the sprockets 2L and 2R. Therefore, when thesprockets are rotationally driven, the crawler belts 3L and 3R aredriven, and this propels the bulldozer 1. The bulldozer 1 furtherincludes left and right rotational speed sensors 42L and 42R that sensethe rotational speed of the left and right output shafts 21L and 21R,for the purpose of sensing the vehicle speed of the bulldozer 1, etc. Inthe following description, for the sake of convenience, the sprockets 2Land 2R may also be called drive wheels.

The left and right steering clutches 12L and 12R are respectivelydisposed between the transmission 17 and the sprockets 2L and 2R, andare hydraulic clutches that can be hydraulically switched betweenengaged and disengaged states. The supply and discharge of hydraulicfluid to and from the steering clutches 12L and 12R are controlled bysteering clutch pressure control valves 27L and 27R.

Here, if the left steering clutch 12L is in its engaged state, powerfrom the second bevel gear 19 is transmitted to the left sprocket 2L. Ifthe left steering clutch 12L is in its disengaged state, power from thesecond bevel gear 19 is cut off by the left steering clutch 12L, and isnot transmitted to the left sprocket 2L. If the right steering clutch12R is in its engaged state, power from the second bevel gear 19 istransmitted to the right sprocket 2R. If the right steering clutch 12Ris in its disengaged state, power from the second bevel gear 19 is cutoff by the right steering clutch 12R, and is not transmitted to theright sprocket 2R.

The left and right steering brakes 13L and 13R are respectively disposedbetween the left and right steering clutches 12L and 12R and thesprockets 2L and 2R, and are hydraulic brakes that can be hydraulicallyswitched between a braking state and a non-braking state. The supply anddischarge of hydraulic fluid to and from the left and right steeringbrakes 13L and 13R are controlled by brake pressure control valves 28Land 28R.

The output rotation of the left steering clutch 12L, that is, therotation of the left sprocket 2L, can be braked by putting the leftsteering brake 13L in a braking state. The output rotation of the rightsteering clutch 12R, that is, the rotation of the right sprocket 2R, canbe braked by putting the right steering brake 13R in a braking state.

With the above configuration, in a state in which the left steeringclutch 12L is disengaged and the left steering brake 13L is braking, ifthe right steering clutch 12R is engaged and the right sprocket 2R isrotationally driven, the bulldozer 1 will turn to the left. Conversely,in a state in which the right steering clutch 12R is disengaged and theright steering brake 13R is braking, if the left steering clutch 12L isengaged and the left sprocket 2L is rotationally driven, the bulldozer 1will turn to the right.

Configuration Around Steering Clutches

Referring to FIG. 15, a first left rotary member 56L is linked to oneside of the left steering clutch 12L, and a second left rotary member57L is linked to the other side. That is, the power transmission device11 includes the first left rotary member 56L and the second left rotarymember 57L.

The first left rotary member 56L is engaged by spline mating with thelateral shaft 20. The first left rotary member 56L rotates integrallywith the second bevel gear 19 and the lateral shaft 20. The second leftrotary member 57L is engaged by spline mating with the left output shaft21L linked to the left final reduction gear 22L. Therefore, the leftdrive unit 4L is driven by the second left rotary member 57L. The secondleft rotary member 57L is rotatably supported by a first left bearing66L disposed on the first left rotary member 56L. The power transmissiondevice 11 includes the first left bearing 66L.

A first left support member 58L is linked to one side of the leftsteering brake 13L, and the second left rotary member 57L is linked tothe other side. The first left support member 58L is fixed to a housing50 of the power transmission device 11. The first left support member58L and the housing 50 are collectively referred to here as a supportmember. The power transmission device 11 includes the support member.

A second left bearing 67L and a third left bearing 68L are attached onthe support member. The power transmission device 11 includes the secondleft bearing 67L and the third left bearing 68L. The second left bearing67L and the third left bearing 68L rotatably support the second leftrotary member 57L and the left output shaft 21L. Consequently, thesupport member rotatably supports the second left rotary member 57L andthe left output shaft 21L. The second left rotary member 57L and theleft output shaft 21L rotate integrally.

The bulldozer 1 includes a hydraulic fluid supply channel 60 thatsupplies hydraulic fluid to the power transmission device 11, and morespecifically to the left and right steering clutch 12L and 12R and theleft and right steering brakes 13L and 13R inside the support member andthe second left rotary member 57L. The hydraulic fluid supply channel 60includes a first left hydraulic fluid supply channel 61L, a second lefthydraulic fluid supply channel 62L, and a left connected part 63L. Thefirst left hydraulic, fluid supply channel 61L is formed inside thesecond left rotary member 57L, and connects to the left steering clutch12L. The second left rotary member 57L includes the first left hydraulicfluid supply channel 61L. The first left hydraulic fluid supply channel61L supplies hydraulic fluid to the left steering clutch 12L.

The second left hydraulic fluid supply channel 62L is formed inside thesupport member, that is, on the outside of the second left rotary member57L. The second left hydraulic fluid supply channel 62L supplieshydraulic fluid to the first left hydraulic fluid supply channel 61L.The left connected part 63L connects the first left hydraulic fluidsupply channel 61L to the second left hydraulic fluid supply channel62L. Also, the hydraulic fluid supply channel 60 separately includes abraking left supply channel 69L that connects to the left steering brake13L. In FIG. 15, the braking left supply channel 69L is disposedoverlapping the first left hydraulic fluid supply channel 61L, and thebraking left supply channel 69L is located on the back side of the firstleft hydraulic fluid supply channel 61L.

The left connected part 63L is disposed in a gap between the supportmember and the second left rotary member 57L. The power transmissiondevice 11 includes a first left sealing ring 64L and a second leftsealing ring 65L. The first left sealing ring 64L and the second leftsealing ring 65L are mounted adjacent to the left connected part 63L sothat hydraulic fluid will not leak out from the left connected part 63L.More specifically, the first left sealing ring 64L is mounted on thesurface of the second left rotary member 57L on the side closer to thesecond bevel gear 19 than the left connected part 63L (on the inside ofthe vehicle). The second left sealing ring 65L is mounted on the surfaceof the second left rotary member 57L on the side closer to the leftfinal reduction gear 22L than the left connected part 63L (on theoutside of the vehicle).

The gap between the support member and the second left rotary member 57Lis sealed by the first left sealing ring 64L and the second left sealingring 65L. Therefore, the left connected part 63L is formed by the gapbetween the support member and the second left rotary member 57L, whichis sealed by the first left sealing ring 64L and the second left sealingring 65L. In the example in FIG. 15, the shaft diameters of the firstleft sealing ring 64L and the second left sealing ring 65L are equal,but this is not necessarily the case.

FIG. 15 shows only the first left rotary member 56L, the second leftrotary member 57L, the first left support member 58L, the first lefthydraulic fluid supply channel 61L, the second left hydraulic fluidsupply channel 62L, the left connected part 63L, the braking left supplychannel 69L, the first left sealing ring 64L, the second left sealingring 65L, the first left bearing 66L, the second left bearing 67L, andthe third left bearing 68L.

However, the hydraulic fluid supply channel 60 also includes a firstright hydraulic fluid supply channel 61R, a second right hydraulic fluidsupply channel 62R, a right connected part 63R, and a braking rightsupply channel 69R, which are connected to the right steering clutch 12Ror the right steering brake 13R. The first right hydraulic fluid supplychannel 61R, the second right hydraulic fluid supply channel 62R, theright connected part 63R, and the braking right supply channel 69Rrespectively have the same structure as the first left hydraulic fluidsupply channel 61L, the second left hydraulic fluid supply channel 62L,the left connected part 63L, and the braking left supply channel 69L.

Also, the power transmission device 11 includes a first right rotarymember 56R, a second right rotary member 57R, and a first right supportmember 58R, which are connected to the left steering clutch 12R or theright steering brake 13R. The first right rotary member 56R, the secondright rotary member 57R, and the first right support member 58R have thesame structure as the first left rotary member 56L, the second leftrotary member 57L, and the first left support member 58L.

The power transmission device 11 includes a first right sealing ring64R, a second right sealing ring 65R, a first right bearing 66R, asecond right bearing 67R, and a third right bearing 68R, which areconnected to the first right rotary member 56R, the second right rotarymember 57R, or the first right support member 58R. The first rightsealing ring 64R, the second right sealing ring 65R, the first rightbearing 66R, the second right bearing 67R, and the third right bearing68R respectively have the same structure as the first left sealing ring64L, the second left sealing ring 65L, the first left bearing 66L, thesecond left bearing 67L, and the third left bearing 68L.

The left steering clutch 12L is a wet multi-plate type, and includesclutch disks 51 and a clutch piston 52. With the left steering clutch12L, when the hydraulic pressure of the hydraulic fluid from the firstleft hydraulic fluid supply channel 61L is applied to the clutch piston52, the clutch disks 51 are joined by hydraulic pressure and power istransmitted. Therefore, the left steering clutch 12L is engaged whenhydraulic fluid supplied from the hydraulic fluid supply channel 60 tothe left steering clutch 12L.

Saying that the left steering clutch 12L is engaged means that apressure equal to or greater than a specific holding pressure, which isthe pressure at which torque within the designed range can betransmitted without clutch slippage, is being supplied to the leftsteering clutch 12L. Normally, when the left steering clutch 12L isengaged, the pressure of the hydraulic fluid applied to the leftsteering clutch 12L is a first pressure that is equal to or greater thanthe holding pressure. The pressure exerted on the left steering clutch12L (clutch pressure) increases in proportion to the hydraulic pressuresupplied to the left steering clutch 12L. In FIG. 15, only the leftsteering clutch 12L is shown, but the right steering clutch 12R has thesame structure.

The left steering brake 13L is a wet multi-plate type, and includesbrake disks 53, a brake piston 54, and a plate spring 55. The leftsteering brake 13L is a so-called negative brake, with which the brakedisks 53 are pressed and a braking state is produced by the biasingforce of the plate spring 55 in a state in which the hydraulic pressureof the hydraulic fluid from the braking left supply channel 69L is notbeing applied to the brake piston 54.

When the hydraulic pressure of the hydraulic fluid from the braking leftsupply channel 69L is applied to the brake piston 54, the brake piston54 causes the brake disks 53 to move apart against the biasing force ofthe plate spring. This puts the left steering brake 13L in a releasedstate. In FIG. 15, only the left steering brake 13L is shown, but theright steering brake 13R has the same structure.

Here, when braking is produced by the left steering brake 13L, the leftsteering clutch 12L is released. At this point the second left rotarymember 57L is stationary along with the support member. Therefore, therotational speed of the first left sealing ring 64L and the second leftsealing ring 65L is zero, and there is little hydraulic pressure appliedto the first left sealing ring 64L and the second left sealing ring 65L.The PV value is the product of the surface pressure P applied to therotary member and the slipping velocity V. The same can be the of theright steering brake 13R.

Meanwhile, when the left steering brake 13L is released and the leftsteering clutch 12L is engaged, the second left rotary member 57Lrotates along with the first left rotary member 56L under the power fromthe transmission 17. The support member, however, is stationary.Therefore, the slipping velocities V of the first left sealing ring 64Land the second left sealing ring 65L respectively correspond to theproduct of multiplying the shaft diameters of the first left sealingring 64L and the second left sealing ring 65L by the circumference ratioand the rotational speed of the second left rotary member 57L.

When no slip is occurring at the left steering clutch 12L, therotational speed of the second left rotary member 57L is equal to therotational speed of the first left rotary member 56L. Furthermore, sincethe left steering clutch 12L is engaged, the surface pressure P appliedto the first left sealing ring 64L and the second left sealing ring 65Lis normally the above-mentioned first pressure. When the rotationalspeed of the second left rotary member 57L rises, or when the clutchpressure of the left steering clutch 12L increases, there is an increasein the PV values of the first left sealing ring 64L and the second leftsealing ring 65L. The same can be the of the right steering clutch 12R.

Configuration for Output Control

This bulldozer 1 has a controller 30 (see FIG. 14). The controller 30includes a CPU or other such computation device, and a RAM, ROM, orother such memory device. The controller 30 is connected to a steeringlever 46 and an upshift/downshift button 47, which are housed in the cab7.

The steering lever 46 is used to switch the bulldozer 1 between forwardand reverse movement and to switch its turning direction. Theupshift/downshift button 47 is used by the operator to shift the gear ofthe transmission 17. The controller 30 receives a signal from thesteering lever 46 or the upshift/downshift button 47 and shifts the gearof the transmission 17 and controls the pressure control valves 27L,27R, 28L, and 28R.

The controller 30 decides the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60. More specifically, the controller 30decides the pressure PL of the hydraulic fluid inside first lefthydraulic fluid supply channel 61L, the second left hydraulic fluidsupply channel 62L, and the left connected part 63L. The controller 30outputs a command corresponding to the decided pressure PL to the leftsteering clutch pressure control valve 27L. Thus, the controller 30performs control so that the pressure of the hydraulic fluid inside thefirst left hydraulic fluid supply channel 61L, the second left hydraulicfluid supply channel 62L, and the left connected part 63L will be thedecided pressure PL.

Similarly, the controller 30 decides the pressure PL of the hydraulicfluid inside first right hydraulic fluid supply channel 61R, the secondright hydraulic fluid supply channel 62R, and the right connected part63R. The controller 30 then outputs a command corresponding to thedecided pressure PR to the right steering clutch pressure control valve27R of the right steering clutch 12R. The steering clutch pressurecontrol valves 27L and 27R receive the command from the controller 30and control the pressure of the hydraulic fluid in the hydraulic fluidsupply channel 60. Thus, the controller 30 performs control so that thatpressure of the hydraulic fluid inside the first right hydraulic fluidsupply channel 61R, the second right hydraulic fluid supply channel 62R,and the right connected part 63R will be the decided pressure PR.

The controller 30 further connects the left rotational speed sensor 42Lthat senses the rotational speed of the left output shaft 21L, the rightrotational speed sensor 42R that senses the rotational speed of theright output shaft 21R, and a monitor 49 that displays the speed of thebulldozer 1 and the state of various devices installed in the bulldozer1. The power transmission device 11 includes the left rotational speedsensor 42L and the right rotational speed sensor 42R. In thisembodiment, the controller 30 outputs information to notify the workerabout replacing the sealing rings 64L, 65L, 64R, and 65R on the basis ofthe rotational speed of the left output shaft 21L and the rotationalspeed of the right output shaft 21R.

FIG. 16 is a block diagram of the detailed configuration of thecontroller 30 according to the first embodiment. The controller 30includes a computer 31, an information output component 32, and a memorycomponent 39. Typically, programs and data for executing the variousfunctions of the computer 31 and the information output component 32 arestored in the memory device. When the computation device executes theprogram, the controller 30 executes the various functions of thecomputer 31 and the information output component 32. The memorycomponent 39 is realized by a part of the above described memory device.The controller 30 may also be realized by an integrated circuit.

The rotational speeds of the left and right rotational speed sensors 42Land 42R are inputted to the computer 31. The rotational speed φLmeasured by the left rotational speed sensor 42L corresponds to therotational speed of the first left sealing ring 64L and the second leftsealing ring 65L. The rotational speed φR measured by the rightrotational speed sensor 42R corresponds to the rotational speed of thefirst right sealing ring 64R and the second right sealing ring 65R. Thecomputer 31 further acquires the values for the pressures PL and PRdecided by the controller 30. The pressure PL corresponds to thehydraulic pressure exerted on the sealing rings 64L and 65L. Thepressure PR corresponds to the hydraulic pressure exerted on the sealingrings 64R and 65R.

The computer 31 finds determination basis data related to the hydraulicpressures PL and PR exerted on the sealing rings, the times when thepressure was controlled to the hydraulic pressures PL and PR, therotational speeds φL and φR of the sealing rings 64L, 65L, 64R, and 65R,and the times when the pressure was controlled to the hydraulicpressures PL and PR while the sealing rings 64L, 65L, 64R, and 65R wererotating. This determination basis data is either data that includes thehydraulic pressures PL and PR, the rotational speeds φ1, and φR, and thetimes when the pressure was controlled to the hydraulic pressures PL andPR while the sealing rings 64L, 65L, 64R, and 65R were rotating, or datathat includes values derived from the above values.

The determination basis data may, for example, be a list that includesthe hydraulic pressures PL and PR at various times, and the rotationalspeeds φL and φR of the sealing rings 64L, 65L, 64R, and 65R at thesevarious times. The determination basis data may also, for example, be alist that includes the products of multiplying the hydraulic pressuresPL and PR by the rotational speeds φL and φR of the sealing rings 64L,65L, 64R, and 65R at various times. The determination basis data mayalso, for example, be a first integration value obtained by the timeintegration of the products of multiplying the hydraulic pressures PLand PR by the rotational speeds φL and φR at the same times as thehydraulic pressures PL and PR.

The computer 31 stores the determination basis data in the memorycomponent 39. If the times when the pressure is controlled to thehydraulic pressures PL and PR and the times when the rotational speedsφL and φR are sensed are not synchronized, the computer 31 mayseparately find the hydraulic pressures PL and PR or the rotationalspeeds φL and φR that have been time synchronized by a linearinterpolation or another such method. If a first integration value isfound as determination basis data, the computer 31 may calculate thefirst integration value by adding a value stored in the memory component39 to the product of multiplying the elapsed time since the time of theprevious measurement by the product of the hydraulic pressure applied toeach sealing ring and the rotational speed of each sealing ring.

The predicated amount of wear W of the sealing rings ML, 65L, 64R, and65R is calculated from the following formula (1).

W=k·P·V·T  (1)

k: coefficient of friction (established according to the material, etc.,of the sealing rings 64L, 65L, 64R, and 65R)P: surface pressure, that is, the hydraulic pressure applied to thesealing rings 64L, 65L, 64R, and 65RV: slipping velocity of the sealing rings ML, 65L, 64R, and 65RT: time

Therefore, when the coefficient of friction, circumference ratio, andshaft diameter of the sealing rings ML, 65L, 64R, and 65R are multipliedby the first integration value, the result is the predicated amount ofwear of the sealing rings ML, 65L, 64R, and 65R. The computer 31 maycalculate the predicated amount of wear W as determination basis data.

The information output component 32 refers to the memory component 39 tofind the predicated amount of wear from the first integration value orthe determination basis data as needed, and if the predicated amount ofwear exceeds a specific threshold, maintenance information about thesealing rings 64L, 65L, 64R, and 65R is outputted to the monitor 49.This maintenance information is information used for recommending thatthe sealing rings 64L, 65L, 64R, and 65R be replaced, or that theabove-mentioned pressures PL and PR be checked to make sure they arenormal or not. This threshold is preset on the basis of the usableamount of wear for the sealing rings 64L, 65L, 64R, and 65R, and isstored in the memory component 39. The output method of the monitor 49is not just displaying this information on a screen, and may also beaudio output from a speaker attached to the monitor 49, or both screenoutput and audio output may be used.

Next, a method for monitoring the bulldozer 1 will be described throughreference to a flowchart. FIG. 17 is a flowchart of a method formonitoring the bulldozer in the first embodiment. First, in step 1, thecontroller 30 decides the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60. More specifically, the controller 30decides the pressure PL of the hydraulic fluid in a first left hydraulicfluid supply channel 61L. Similarly, the controller 30 decides thepressure PR of the hydraulic fluid in a first right hydraulic fluidsupply channel 61R. The controller 30 outputs a command corresponding tothe decided pressures PL and PR to the steering clutch pressure controlvalves 27L and 27R, and controls the pressure of the hydraulic fluid inthe hydraulic fluid supply channel 60 so that it will be the decidedpressures PL and PR.

In step 2, the left and right rotational speed sensors 42L and 42Rmeasure the rotational speeds φL and φR of the sealing rings 64L, 65L,64R, and 65R. The computer 31 acquires the measured rotational speeds φLand φR and the times at which these rotational speeds φL and φR weremeasured. In step 3, the computer 31 acquires the pressures PL and PRdecided by the controller 30, and the times controlled by thesepressures PL and PR. The computer 31 then finds the above-mentioneddetermination basis data.

In step 4, the information output component 32 determines whether or notthe predicated amount of wear of the sealing rings obtained from thedetermination basis data exceeds a specific threshold. If the predicatedamount of wear does not exceed the specific threshold (No in step 4),the flow returns to step 1. If the predicated amount of wear does exceedthe specific threshold (Yes in step 4), in step 5 the information outputcomponent 32 outputs maintenance information about the sealing rings64L, 65L, 64R, and 65R to the monitor 49. Upon completion of step 5, theflow returns to step 1.

Second Embodiment

In the first embodiment, the bulldozer 1 outputs maintenance informationabout the sealing rings 64L, 65L, 64R, and 65R to the monitor 49.However, maintenance information about the sealing rings 64L, 65L, 64R,and 65R may instead be outputted to an external monitoring device. Inthe second embodiment, a monitoring system that includes a monitoringdevice such as this will be described.

FIG. 18 shows the overall configuration of a monitoring system 100according to the second embodiment. The monitoring system 100 includes abulldozer 1 a and a monitoring device 8. The bulldozer 1 a is able tooutput information to the external monitoring device 8. The monitoringdevice 8 is provided on the outside of the bulldozer 1 a, and is able toinput information from the bulldozer 1 a. With the bulldozer 1 a, thecomponents other than the controller 30 a and an external outputcomponent 48 (discussed below) are all the same as in the firstembodiment, and will not be described again.

The bulldozer 1 a according to the second embodiment includes thecontroller 30 a instead of the controller 30, and further includes theexternal output component 48. The controller 30 a differs from thecontroller 30 in that it does not include the information outputcomponent 32, but the rest of the configuration of the controller 30 ais the same as that of the controller 30.

The external output component 48 outputs the determination basis datacalculated by the computer 31 in a format that can be inputted by themonitoring device 8. The external output component 48 is, for example, acommunication interface of a communication means 9 used forcommunicating with the monitoring device 8. The communication means 9may involve wired communication, or wireless communication such assatellite communication or communication by a portable telephonenetwork.

The external output component 48 may be such that, of the determinationbasis data, the data related to the hydraulic pressures PL and PRapplied to the sealing rings is outputted separately from the datarelated to the rotational speeds φL and φR. Data related to thehydraulic pressures PL and PR applied to the sealing rings includes thehydraulic pressures PL and PR themselves and the times when the pressurewas controlled to the hydraulic pressures PL and PR. Data related to therotational speeds φL and φR includes the rotational speeds φL and φRthemselves and the times when the rotational speeds φL and φR weresensed.

If there is no need for the times when the pressure was controlled tothe hydraulic pressures PL and PR to be inputted to the monitoringdevice 8 because the times have been preset, or for some other suchreason, then data related to the hydraulic pressures PL and PR appliedto the sealing rings and outputted to the external output component 48may not include the times when the pressure was controlled to thehydraulic pressures PL and PR. The same applies to data related to therotational speeds φL and φR.

The monitoring device 8 is a computer or a dedicated terminal for remotemonitoring. The monitoring device 8 includes a monitor for displayinginformation, a speaker for outputting information as sound, or the like.The monitoring device 8 notifies the operator of the state of thebulldozer 1 a. The monitoring device 8 also includes an input component81 and an output component 83. The input component 81 accepts thedetermination basis data outputted by the external output component 48.The input component 81 is, for example, a communication interface of thecommunication means 9.

If the determination basis data is neither the above-mentioned firstintegration value nor the predicated amount of wear, the outputcomponent 83 may calculate a first integration value from thedetermination basis data, and calculate the predicated amount of wearfrom Formula 1. In the determination basis data, if the times when thepressure was controlled to the hydraulic pressures PL and PR and thetimes when the rotational speeds φL and φR were sensed are notsynchronized, then the output component 83 may use linear interpolationor another such method to separately find hydraulic pressures PL and PRor rotational speeds φL and φR that have been time synchronized. If thedetermination basis data is the above-mentioned first integration value,the output component 83 may calculate the predicated amount of wear onthe basis of Formula 1 from the first integration value. If thepredicated amount of wear exceeds a specific threshold, the outputcomponent 83 outputs the above-mentioned maintenance information aboutthe sealing rings 64L, 65L, 64R, and 65R to the monitor, speakers, etc.,of the monitoring device 8.

Next, a method for monitoring the bulldozer 1 a will be describedthrough reference to a flowchart. FIG. 19 is a flowchart of theoperation of the bulldozer 1 a, out of the monitoring method with themonitoring system 100 according to the second embodiment. FIG. 20 is aflowchart of the operation of the monitoring device 8 a, out of themonitoring method with the monitoring system 100 according to the secondembodiment. Those steps that are the same as in the first embodimentwill be numbered the same as in FIG. 17, and will not be describedagain. In step 6 in FIG. 19, the external output component 48 of thebulldozer 1 a outputs the determination basis data found by the computer31 in a format that can be inputted by the monitoring device 8. Whenstep 6 is finished, the flow returns to step 1.

In step 81 in FIG. 20, the input component 81 of the monitoring device 8accepts the determination basis data outputted by the external outputcomponent 48. In step 84, the output component 83 of the monitoringdevice 8 finds the predicated amount of wear of the sealing rings fromthe determination basis data as needed, and determines whether or notthe predicated amount of wear exceeds a specific threshold.

If the predicated amount of wear does not exceed the specific threshold(No in step 84), the flow returns to step 81. If the predicated amountof wear does exceed the specific threshold (Yes in step 84), in step 85the output component 83 of the monitoring device 8 outputs maintenanceinformation about the sealing rings 64L, 65L, 64R, and 65R to themonitor of the monitoring device 8, a speaker, etc. When step 85 isfinished, the flow returns to step 81.

Third Embodiment

In the first and second embodiments, an example was given in which thepredicated amount of wear W of the sealing rings 64L, 65L, 64R, and 65Rwas proportional to the slipping velocity V and the surface pressure P,but an abnormal amount of friction will be produced if the PV value (theproduct of P and V) exceeds the maximum permissible value set by themanufacturer of the sealing rings 64L, 65L, 64R, and 65R. This abnormalamount of friction is a value that greatly exceeds the value found fromFormula 1. If this abnormal friction state continues for an extendedperiod, the replacement date for the sealing rings 64L, 65L, 64R, and65R found from the predicated amount of wear W will deviate greatly fromthe ideal replacement date. In view of this, in the third embodiment wewill describe a bulldozer that prevents this abnormal friction.

FIG. 21 is a block diagram of a controller 30 b of a bulldozer 1 baccording to the third embodiment. With the bulldozer 1 b, thecomponents other than the controller 30 b are all the same as in thefirst embodiment, and will therefore not be described again. Also, thecontroller 30 b is the same as the controller 30 according to the firstembodiment except that it further includes a pressure controller 33.Therefore, only the pressure controller 33 will be described below. Withthe controller 30 b, typically, a program and data for executing thefunctions of the pressure controller 33 are stored in a memory device. Acomputation device executes the program, and as a result the controller30 b executes the functions of the pressure controller 33. Thecontroller 30 b may be realized by an integrated circuit.

The rotational speeds of the above-mentioned left and right rotationalspeed sensors 42L and 42R are inputted to the pressure controller 33.The pressure controller 33 determines whether or not the rotationalspeed ΦL measured by the left rotational speed sensor 42L is greaterthan the specific rotational speed threshold ΦthL, and whether or notthe rotational speed ΦR measured by the right rotational speed sensor42R is greater than the specific rotational speed threshold ΦthR. Therotational speed threshold ΦthL is predetermined so as to satisfy thefollowing Formula 2, and the rotational speed threshold ΦthR ispredetermined so as to satisfy the following Formula 3.

ΦthL≦MaxPVL/(DL×P1)  (2)

MaxPVL: maximum permissible PV value of first left sealing ring firstleft sealing ring 64L and second left sealing ring 65LDL: shaft diameter of first left sealing ring 64L and second leftsealing ring 65LP1: first pressure

ΦthR≦MaxPVR/(DR×P1)  (3)

MaxPVR: maximum permissible PV value of first right sealing ring 64R andsecond right sealing ring 65RDR: shaft diameter of first right sealing ring 64R and second rightsealing ring 65RP1: first pressure

When the first left sealing ring 64L, the second left sealing ring 65L,the first right sealing ring 64R, and the second right sealing ring 65Rare all the same sealing ring, the right side of Formula 2 and the rightside of Formula 3 are equivalent. In this case, it is preferable ifΦthL=ΦthR.

In the above description, an example is given of a case in which themaximum permissible PV values of the first left sealing ring 64L and thesecond left sealing ring 65L are the same as the maximum permissible PVvalues of the first right sealing ring 64R and the second right sealingring 65R, and the shaft diameters of the first left sealing ring 64L andthe second left sealing ring 65L are the same as the shaft diameters ofthe first right sealing ring 64R and the second right sealing ring 65R.

However, there are also cases when the first left sealing ring 64L andthe second left sealing ring 65L have different maximum permissible PVvalues. Or, the first left sealing ring 64L and the second left sealingring 65L may have different shaft diameters. In such a case,MaxPVL/(DL×P1) may be calculated for each of the first left sealing ring64L and the second left sealing ring 65L, and the rotational speedthreshold ΦthL set for the first left sealing ring 64L and the secondleft sealing ring 65L so as to be equal to or less than the lower of thecalculated values.

Similarly, there are also cases when the first right sealing ring 64Rand the second right sealing ring 65R have different maximum permissiblePV values. Or, the first right sealing ring 64R and the second leftsealing ring second right sealing ring 65R may have different shaftdiameters. In such a case, MaxPVR/(DR×P1) may be calculated for each ofthe first right sealing ring 64R and the second right sealing ring 65R,and the rotational speed threshold ΦthR set for the first right sealingring 64R and the second right sealing ring 65R so as to be equal to orless than the lower of the calculated values.

The pressure controller 33 decides the pressure of the hydraulic fluidin the hydraulic fluid supply channel 60 so as to keep the PV value ofthe left and right steering clutches 12L and 12R within the permissibleusage range. More specifically, the pressure controller 33 decides thepressure of the hydraulic fluid in the first left hydraulic fluid supplychannel 61L, the second left hydraulic fluid supply channel 62L, and theleft connected part 63L so as to keep the PV value of the left steeringclutch 12L within the permissible usage range. The pressure controller33 then outputs a command corresponding to the decided engagementpressure to the left steering clutch pressure control valve 27L. Thepressure controller 33 performs control so that the clutch pressure ofthe left steering clutch I 2L will be the decided pressure.

Similarly, the pressure controller 33 decides the pressure of thehydraulic fluid in the first right hydraulic fluid supply channel 61R,the second right hydraulic fluid supply channel 62R, and the rightconnected part 63R so as to keep the PV value of the right steeringclutch 12R within the permissible usage range. The pressure controller33 then outputs a command corresponding to the decided pressure to theright clutch pressure control valve 27R. The pressure controller 33performs control so that the clutch pressure of the right steeringclutch 12R will be the decided pressure. The left and right clutchpressure control valves 27L and 27R receive commands from the pressurecontroller 33, and control the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60.

More specifically, if the rotational speed ΦL is equal to or less thanthe rotational speed threshold ΦthL, and the rotational speed ΦR isequal to or less than the rotational speed threshold ΦthR, the pressurecontroller 33 outputs a command to the left and right clutch pressurecontrol valves 27L and 27R to use the above-mentioned first pressure forthe pressure of the hydraulic fluid in the hydraulic fluid supplychannel 60. The pressure controller 33 performs control so that thepressure of the hydraulic fluid in the hydraulic fluid supply channel 60will be the first pressure.

If the rotational speed DL is greater than the rotational speedthreshold ΦthL, or if the rotational speed ΦR is less than therotational speed threshold ΦthR, the pressure controller 33 outputs acommand to the left and right clutch pressure control valves 27L and 27Rto reduce the pressure of the hydraulic fluid in the hydraulic fluidsupply channel 60 from the first pressure. The pressure controller 33performs control so that the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60 will be reduced from the firstpressure.

More specifically, if the rotational speed ΦL is greater than therotational speed threshold ΦthL, or if the rotational speed ΦR is lessthan the rotational speed threshold ΦthR, the pressure controller 33performs control so that the pressure PL will be reduced so that theproduct of the rotational speed ΦL and the pressure PL exerted on theleft steering clutch 12L will not exceed the specific upper limit UlimL.

Similarly, if the rotational speed ΦL is greater than the rotationalspeed threshold ΦthL, or if the rotational speed ΦR is less than therotational speed threshold ΦthR, the pressure controller 33 performscontrol so that the pressure PR will be reduced so that the product ofthe rotational speed ΦR and the pressure PR exerted on the rightsteering clutch 12R will not exceed the specific upper limit UlimR.These specific upper limit UlimL and UlimR are determined from Formulas4 and 5.

UlimL=ΦthL×P1  (4)

ΦthL: rotational speed threshold determined so as to satisfy Formula 2P1: first pressure

UlimR=ΦthR×P1  (5)

ΦthR: rotational speed threshold determined so as to satisfy Formula 3P1: first pressure

These upper limits UlimL and UlimR may be called a first product. Thepressure controller 33 can make use of the first product UlimL to setthe target hydraulic pressure PL applied to the left steering clutch 12Las in Formula 6. Similarly, the pressure controller 33 can make use ofthe first product UlimR to set the target hydraulic pressure PR appliedto the right steering clutch 12R as in Formula 7. The pressurecontroller 33 outputs commands to the left and right clutch pressurecontrol valves 27L and 27R to use the target hydraulic pressures PL andPR thus set.

PL=UlimL/MAX(ΦL,ΦR)  (6)

MAX (ΦL, ΦR): rotational speed of the greater of ΦL and ΦR

PR=UlimR/MAX(ΦL,ΦR)  (7)

MAX (ΦL, ΦR): rotational speed of the greater of ΦL and ΦR

If the rotational speed ΦL of the left rotational speed sensor 42L ishigher than the rotational speed threshold ΦthL, the pressure controller33 may control the pressure PL of the hydraulic fluid on the leftsteering clutch 12L regardless of the rotational speed ΦR of the rightrotational speed sensor 42R. Similarly, if the rotational speed ΦR ofthe right rotational speed sensor 42R is higher than the rotationalspeed threshold ΦthR, the pressure controller 33 may control thepressure PR of the hydraulic fluid on the right steering clutch 12Rregardless of the rotational speed ΦL of the left rotational speedsensor 42L.

In this case, the pressure controller 33 preferably performs control sothat the clutch pressure of the left steering clutch 12L will be thehydraulic pressure PL found from Formula 8. Similarly, the pressurecontroller 33 preferably performs control so that the clutch pressure ofthe right steering clutch 12R will be the hydraulic pressure PR foundfrom Formula 9.

$\begin{matrix}{{PL} = \left\{ \begin{matrix}{P\; 1} & \left( {{VL} \leq {{VthL}}} \right) \\{U\mspace{11mu} \lim \mspace{11mu} {L/{VL}}} & \left( {{VL} > {{VthL}}} \right)\end{matrix} \right.} & (8) \\{{PR} = \left\{ \begin{matrix}{P\; 1} & \left( {{VR} \leq {{VthR}\mspace{11mu} }} \right) \\{U\mspace{11mu} \lim \mspace{11mu} {R/{VR}}} & \left( {{VR} > {{VthR}\mspace{11mu} }} \right)\end{matrix} \right.} & (9)\end{matrix}$

Next, a method for monitoring the bulldozer 1 according to thisembodiment will be described through reference to a flowchart. Theoperation of the above-mentioned pressure controller 33 corresponds tothe detailed operation in step 1 in the first embodiment. FIG. 22 is aflowchart of the detailed operation in step 1 with the bulldozeraccording to the third embodiment.

In step 10, the left rotational speed sensor 42L measures the rotationalspeed ΦL of the left sealing rings 64L and 65L, and the right rotationalspeed sensor 42R measures the rotational speed ΦR of the sealing rings64R and 65R. In step 11, the pressure controller 33 determines whetheror not the rotational speed ΦL measured by the left rotational speedsensor 42L is greater than the specific rotational speed threshold ΦthL.If the rotational speed ΦL is not greater than the rotational speedthreshold ΦthL (No in step 11), in step 12 the pressure controller 33determines whether or not the rotational speed ΦR measured by the rightrotational speed sensor 42R is greater than the specific rotationalspeed threshold ΦthR.

If the rotational speed ΦL is greater than the rotational speedthreshold ΦthL (Yes in step 11), or if the rotational speed ΦR isgreater than the specific rotational speed threshold ΦthR (Yes in step12), in step 13 the pressure controller 33 decides to reduce the clutchpressures PL and PR (the pressure inside the first/right left hydraulicfluid supply channels 61L and 61R) from the first pressure P1. Thepressure controller 33 then performs control so that the pressure of thehydraulic fluid in the hydraulic fluid supply channel 60 will be thedecided pressure.

More specifically, if the rotational speed ΦL is greater than thespecific rotational speed threshold ΦthL, the pressure controller 33decides to reduce the hydraulic pressure PL so that the product of therotational speed ΦL and the hydraulic pressure PL exerted on the leftsteering clutch 12L will not exceed the upper limit value UlimL. Thepressure controller 33 then controls the clutch pressure of the leftsteering clutch 12L to be the decided pressure.

Also, if the rotational speed ΦR is greater than the specific rotationalspeed threshold ΦthR, the pressure controller 33 decides to reduce thehydraulic pressure PR so that the product of the rotational speed ΦR andthe hydraulic pressure PR exerted on the right steering clutch 12R willnot exceed the upper limit value UlimR. The pressure controller 33 thencontrols the clutch pressure of the right steering clutch 12R to be thedecided pressure.

An example of specific processing is shown in FIG. 23. In step 131, thepressure controller 33 makes a decision so that the hydraulic pressurePL exerted on the left steering clutch 12L will be UlimL/MAX (ΦL, ΦR),and the hydraulic pressure PR exerted on the right steering clutch 12Rwill be UlitnR/MAX (ΦL, ΦR). The pressure controller 33 then performscontrol so that the pressure of the hydraulic pressure in the hydraulicfluid supply channel 60 will be the decided pressure.

Another processing example is shown in FIG. 24. In step 132, when therotational speed ΦL is equal to or less than the rotational speedthreshold ΦthL, the pressure controller 33 decides that the hydraulicpressure PL will be the first pressure P1. When the rotational speed ΦLis greater than the rotational speed threshold ΦthL, the pressurecontroller 33 decides that the hydraulic pressure PL will be UlimL/ΦL.The pressure controller 33 then controls the clutch pressure of the leftsteering clutch 12L to be the decided pressure.

When the rotational speed ΦR is equal to or less than the rotationalspeed threshold ΦthR, the pressure controller 33 performs control sothat the hydraulic pressure PR will be the first pressure P1. When therotational speed CDR is greater than the rotational speed thresholdΦthR, the pressure controller 33 decides that the hydraulic pressure PRwill be UlimR/ΦR. The pressure controller 33 then controls the clutchpressure of the right steering clutch 12R to be the decided pressure.

If the rotational speed ΦR is not greater than the rotational speedthreshold ΦthR (No in step 12 in FIG. 22), in step 14 the pressurecontroller 33 decides that clutch pressures PL and PR (the pressure infirst left/right hydraulic fluid supply channels) will be theabove-mentioned first pressure P1. The pressure controller 33 thenperforms control so that the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60 will be the decided pressure.

Features

(1) With the bulldozer 1 according to the first embodiment, the pressureof the hydraulic fluid in the hydraulic fluid supply channel 60 isdecided, and determination basis data is found that is related to thehydraulic pressures PL and PR, the rotational speeds φL and φR, and thetimes when the rotational speeds controlled by the hydraulic pressuresPL and PR during rotation of the sealing rings 64L, 65L, 64R, and 65Rwere sensed. The bulldozer 1 then outputs maintenance information aboutthe sealing rings 64L, 65L, 64R, and 65R from the predicated amount ofwear W obtained from the determination basis data. Consequently, thebulldozer 1 can notify a worker to suitably replace the sealing rings64L, 65L, 64R, and 65R, etc., according to the usage state of thesealing rings 64L, 65L, 64R, and 65R.

(2) With the monitoring system 100 according to the second embodiment,the bulldozer 1 a decides the pressure of the hydraulic fluid in thehydraulic fluid supply channel 60, and finds the above-mentioneddetermination basis data. The bulldozer 1 a then outputs thedetermination basis data in a format that can be inputted by themonitoring device 8. The monitoring device 8 outputs maintenanceinformation about the sealing rings 64L, 65L, 64R, and 65R from thepredicated amount of wear W obtained from the determination basis data.Consequently, a worker can be suitably notified about replacement of thesealing rings 64L, 65L, 64R, and 65R, etc., according to the usage stateof the sealing rings 64L, 65L, 64R, and 65R.

(3) In the third embodiment, when the rotational speeds ΦL and ΦRmeasured by the rotational speed sensors 42L and 42R are greater thanthe rotational speed thresholds ΦthL and ΦthR, the pressure controller33 performs control to reduce the pressures PL and PR so that theproducts of the rotational speeds ΦL and ΦR and the hydraulic pressuresPL and PR do not exceed the upper limit value UlimL and UlimR. Morespecifically, the pressure controller 33 performs control so that thepressure of the hydraulic fluid will be the quotient of dividing thefirst products UlimL and UlimR, which correspond to the products of therotational speed thresholds ΦthL and ΦthR and the first pressure P1, bythe rotational speeds ΦL and ΦR.

Therefore, the PV value of the sealing rings 64L, 65L, 64R, and 65R isreliably controlled so as not to reach the maximum permissible value.Therefore, abnormal wear of the sealing rings 64L, 65L, 64R, and 65R isprevented, and the replacement date for the sealing rings 64L, 65L, 64R,and 65R can be found more accurately.

Modification Examples

The present invention is not limited to or by the above embodiments, andvarious modifications and variations are possible without departing fromthe scope of the present invention.

(a) In the above embodiments, the information output component 32 or theoutput component 83 may output maintenance information about the sealingrings 64L, 65L, 64R, and 65R when a first integration value, rather thanthe predicated amount of wear, exceeds a threshold.

(b) In the above embodiments, an example was given in which two sealingrings were mounted per rotary member, but just one sealing ring, orthree or more, may be mounted per rotary member.

(c) In the above embodiments, a bulldozer was used as an example of awork vehicle, but the present invention can be similarly applied toother work vehicles in which a steering clutch is provided to a powertransmission device.

(d) In the above embodiments, the first left hydraulic fluid supplychannel 61L may be formed in the first left rotary member 56L. In thiscase, the second left hydraulic fluid supply channel 62L is preferablyconnected to the first left rotary member 56L, and the first leftsealing ring 64L and the second left sealing ring 65L are mounted on thesurface of the first left rotary member 56L. Also, the first righthydraulic fluid supply channel 61R may be formed in the first rightrotary member 56R. In this case, the second right hydraulic fluid supplychannel 62R is preferably connected to the first right rotary member56R, and the first right sealing ring 64R and the second right sealingring 65R are mounted on the surface of the first right rotary member56R.

(e) Also, the first left hydraulic fluid supply channel 61L may includea part of the supply channel in the left output shaft 21L or in thelateral shaft 20, and the first right hydraulic fluid supply channel 61Rmay include a part of the supply channel in the right output shaft 21Ror in the lateral shaft 20. In this case, the second left hydraulicfluid supply channel 62L may be connected to the left output shaft 21Lor the lateral shaft 20, and the first left sealing ring 64L and thesecond left sealing ring 65L may be mounted on the surface of the leftoutput shaft 21L or the lateral shaft 20. The second right hydraulicfluid supply channel 62R may be connected to the right output shaft 21Ror the lateral shaft 20, and the first right sealing ring 64R and thesecond right sealing ring 65R may be mounted on the surface of the rightoutput shaft 21R or the lateral shaft 20.

(f) In the second embodiment, the external output component 48 mayoutput determination basis data to a removable storage medium that canbe written to by the bulldozer 1 a and that can be read by themonitoring device 8. The storage medium is, for example, a CD-ROM, aDVD, a BD, or another such optical disk memory, a memory card, a USBmemory, or another portable memory. In this case, the communicationmeans 9 may be eliminated. Also, the determination basis data may bestored in the storage medium instead of being stored in the memorycomponent 39.

(g) The external output component 48 may display determination basisdata in a text format on the monitor 49 of the bulldozer 1 a. Also, theinput component 81 of the monitoring device 8 may be a keyboard, amouse, a touch panel, or another such input device. In this case, aworker performing maintenance on the bulldozer 1 a may read thedetermination basis data displayed on the monitor 49 and use the inputcomponent 81 to input the read determination basis data to themonitoring device 8. Here again, the communication means 9 may beomitted.

(h) In the third embodiment, an example was given in which the pressurecontroller 33 was added to the bulldozer 1 of the first embodiment, butthe pressure controller 33 may also be added to the bulldozer 1 a of thesecond embodiment.

(i) In the third embodiment, step 2 may be eliminated, and therotational speeds ΦL and ΦR acquired in step 10 may be used instead toderive the determination basis data.

(C)

First Embodiment

FIG. 25 shows a bulldozer 1 that is an example of a tracked workvehicle. As shown in FIGS. 25 and 26, the bulldozer 1 includes driveunits 4L and 4R respectively having sprockets 2L and 2R and crawlerbelts 3L and 3R, a blade 5 provided at the front of the vehicle, and aripper device 6 provided at the rear of the vehicle. This bulldozer 1can perform work such as dozing with the blade 5, or work such ascrushing or excavation with the ripper device 6.

The bulldozer 1 further includes a cab 7 above the left and right driveunits 4L and 4R. The cab 7 is equipped with a seat in which the operatorsits, various kinds of control lever, a vehicle speed setting switch,pedals, gauges, and so forth. In the following description, the “forwardand backward direction” means the forward and backward direction of thebulldozer 1. In other words, the forward and backward direction meansthe forward and backward direction as seen by the operator seated in thecab 7. Also, the left and right direction or “to the side” refers to thevehicle width direction of the bulldozer 1. The left and rightdirection, the vehicle width direction, and “to the side” all refer tothe left and right directions as seen by the above-mentioned operator.

Configuration of Power Transmission System

As shown in FIG. 26, this bulldozer 1 includes an engine 10, a powertransmission device 11 that transmits power from the engine 10 to theleft and right drive units 4L and 4R, left and right steering clutches12L and 12R, and left and right steering brakes 13L and 13R. The powertransmission device 11 includes a torque converter 16 and a transmission17.

The power from the engine 10 is transmitted to a power takeoff 15. Thepower takeoff 15 diverts part of the power from the engine 10 tohydraulic pumps or the like that generate power for the blade 5 and theripper device 6, and transmits the rest of the power to the torqueconverter 16. The torque converter 16 transmits power through a fluid.The output shaft of the torque converter 16 is linked to the input shaftof the transmission 17, and power is transmitted from the torqueconverter 16 to the transmission 17. The transmission 17 changes thespeed of the rotary motion of the engine. The power outputted from thetransmission 17 is transmitted through a first bevel gear 18 and asecond bevel gear 19 to a lateral shaft 20.

The power transmitted to the lateral shaft 20 goes through the leftsteering clutch 12L and a left final reduction gear 22L, and istransmitted to the left sprocket 2L. Also, the power transmitted to thelateral shaft 20 goes through the right steering clutch 12R and a rightfinal reduction gear 22R, and is transmitted to the right sprocket 2R.The crawler belts 3L and 3R are wound around the sprockets 2L and 2R.Therefore, when the sprockets are rotationally driven, the crawler belts3L and 3R are driven, and this propels the bulldozer 1. In the followingdescription, the sprockets 2L and 2R may be called drive wheels for thesake of description.

The left and right steering clutches 12L and 12R are respectivelydisposed between the power transmission device 11 and the left and rightsprockets 2L and 2R, and are hydraulic clutches that can behydraulically switched between engaged and disengaged states. The supplyand discharge of hydraulic fluid to and from these steering clutches 12Land 12R are controlled by steering clutch control valves 27L and 27R.

Saying that the left and right steering clutches 12L and 12R are engagedmeans that a pressure equal to or greater than a specific holdingpressure, at which torque within the designed range can be transmittedwithout clutch slippage, is being supplied to the steering clutches 12Land 12R. The pressure applied to the steering clutches 12L and 12R(clutch pressure) rises in proportion to the hydraulic pressure suppliedto the steering clutches 12L and 12R.

Here, if the left steering clutch 12L is in its engaged state, powerfrom the second bevel gear 19 is transmitted to the left sprocket 2L. Ifthe left steering clutch 12L is in its disengaged state, power from thesecond bevel gear 19 is cut off by the left steering clutch 12L, and isnot transmitted to the left sprocket 2L. If the right steering clutch12R is in its engaged state, power from the second bevel gear 19 istransmitted to the right sprocket 2R. If the right steering clutch 12Ris in its disengaged state, power from the second bevel gear 19 is cutoff by the right steering clutch 12R, and is not transmitted to theright sprocket 2R.

The left and right steering brakes 13L and 13R are respectively disposedbetween the left and right steering clutches 12L and 12R and the leftand right sprockets 2L and 2R, and are hydraulic brakes that can behydraulically switched between a braking state and a non-braking state.The supply and discharge of hydraulic fluid to and from the left andright steering brakes 13L and 13R are controlled by brake control valves28L and 28R.

The output rotation of the left steering clutch 12L, that is, therotation of the left sprocket 2L, can be braked by putting the leftsteering brake 13L in a braking state. The output rotation of the rightsteering clutch 12R, that is, the rotation of the right sprocket 2R, canbe braked by putting the right steering brake 13R in a braking state.

With the above configuration, in a state in which the left steeringclutch 12L is disengaged and the left steering brake 13L is braking, ifthe right steering clutch 12R is engaged and the right sprocket 2R isrotationally driven, the bulldozer 1 will turn to the left. Conversely,in a state in which the right steering clutch 12R is disengaged and theright steering brake 13R is braking, if the left steering clutch 12L isengaged and the left sprocket 2L is rotationally driven, the bulldozer 1will turn to the right.

Configuration for Output Control

This bulldozer 1 has a controller 30. The controller 30 includes a CPUor other such computation device, and a RAM, ROM, or other such memorydevice. The controller 30 is connected to a steering lever 48 and anupshift/downshift button 49, which are housed in the cab 7. The steeringlever 48 is used to switch the bulldozer 1 between forward and reversemovement and to switch its turning direction. The upshift/downshiftbutton 49 is used by the operator to shift the gear of the transmission17.

The controller 30 receives a signal from the steering lever 48 or theupshift/downshift button 49 and shifts the gear of the transmission 17and controls the control valves 27L, 27R, 28L, and 28R.

FIG. 27 is a block diagram of the detailed configuration of thecontroller 30 according to the first embodiment. The controller 30 has astall state determination component 60 and a steering clutch controller70. Typically, programs and data for executing the various functions ofthe stall state determination component 60 and the steering clutchcontroller 70 are stored in a memory device. When the computation deviceexecutes the program, the controller 30 executes the various functionsof the stall state determination component 60 and the steering clutchcontroller 70. The controller 30 may also be realized by an integratedcircuit.

The stall state determination component 60 determines whether or not oneor both of the left and right sprockets 2L and 2L are in a stall state.“Stall state” refers to a state in which the sprockets 2L and 2R cannotbe sufficiently rotated even though the crawler belts 3L and 3R are notslipping with respect to the ground. It means that when the clutchpressure of the steering clutches 12L and 12R is equal to or greaterthan a first pressure and the stall state determination component 60determines that there is no stall state, the steering clutches 12L and12R are both engaged and are not slipping. The first pressure is apressure equal to or greater than the above-mentioned holding pressure.The first pressure will be discussed in detail below.

The stall state determination component 60 includes a speed ratiocomputer 61 and a vehicle speed sensor 62. A first rotational speed froma first rotational speed sensor 52, and a second rotational speed from asecond rotational speed sensor 53 are inputted to the speed ratiocomputer 61.

The first rotational speed sensor 52 senses the first rotational speed,which is the rotational speed of the input shaft of the powertransmission device 11. The input shaft of the power transmission device11 is, for example, the input shaft of the torque converter 16. Thefirst rotational speed sensor 52 is a rotational speed sensor 40 thatsenses the rotational speed of the input shaft of the torque converter16 (see FIG. 26).

The second rotational speed sensor 53 senses the second rotationalspeed, which is the rotational speed of the output shaft of the powertransmission device 11. The output shaft of the power transmissiondevice 11 is, for example, the output shaft of the transmission 17. Thesecond rotational speed sensor 53 is a rotational speed sensor 41 thatsenses the rotational speed of the output shaft of the transmission 17(see FIG. 26). The second rotational speed sensor 53 may also be arotational speed sensor that senses the rotational speed of the outputshaft of the torque converter 16.

The speed ratio computer 61 computes the speed ratio, which is the ratioof the second rotational speed to the first rotational speed. If thesecond rotational speed sensor 53 is a rotational speed sensor thatsenses the rotational speed of the output shaft of the transmission 17,then the speed ratio computed by the speed ratio computer 61 is theproduct of multiplying the reduction ratio of the transmission 17 by thespeed ratio of the input shaft and output shaft of the torque converter16 (input/output speed ratio). If the second rotational speed sensor 53is a rotational speed sensor that senses the rotational speed of theoutput shaft of the torque converter 16, then the speed ratio computedby the speed ratio computer 61 is the input/output speed ratio of thetorque converter 16.

The vehicle speed sensor 62 senses the vehicle speed of the bulldozer 1.The rotational speeds of the rotational speed sensors 42L and 42R (seeFIG. 26) that sense the rotational speeds of the output shafts of thesteering clutches 12L and 12R, for example, are inputted to the vehiclespeed sensor 62. The vehicle speed sensor 62 finds the movement speed VLof the left drive unit 4L by multiplying the circumference ratio and thediameter of the left sprocket 2L and the reduction ratio of the leftfinal reduction gear 22L by the rotational speed of the output shaft ofthe left steering clutch 12L.

Similarly, the vehicle speed sensor 62 finds the movement speed VR ofthe right drive unit 4R by multiplying the circumference ratio and thediameter of the right sprocket 2R and the reduction ratio of the rightfinal reduction gear 22R by the rotational speed of the output shaft ofthe right steering clutch 12R. The vehicle speed sensor 62 finds theaverage of the movement speed VL of the left drive unit 4L and themovement speed VR of the right drive unit 4R as the vehicle speed of thebulldozer 1.

The method for sensing the vehicle speed of the bulldozer 1 is notlimited to the above example. For instance, the bulldozer 1 may beequipped with a GPS receiver, and the vehicle speed sensor 62 may sensethe vehicle speed of the bulldozer 1 on the basis of the amount ofchange per unit of time in the position of the bulldozer 1 sensed byGPS. Also, the vehicle speed sensor 62 may sense the vehicle speed ofthe bulldozer 1 by means of a sensor attached to the left and rightdrive units 4L and 4R.

The gear sensed by a gear sensor 51 is also inputted to the stall statedetermination component 60. The gear sensor 51 is connected to theupshift/downshift button 49, and senses the gear of the powertransmission device 11 set by operator input. Also, the controller 30may shift the gear of the transmission 17 automatically. In this case,the gear sensor 51 preferably senses the gear of the transmission 17that has been automatically shifted.

If the gear sensed by the gear sensor 51 is first gear, the speed ratiocomputed by the speed ratio computer 61 is equal to or less than a firstspeed ratio, and the vehicle speed is equal to or less than a specificfirst speed, the stall state determination component 60 determines thatthe sprockets 2L and 2R are both in a stall state. This first speedratio and first speed are suitably established according to size of thebulldozer 1, the type of the engine 10, and so on.

If the second rotational speed sensor 53 is a rotational speed sensorthat senses the rotational speed of the output shaft of the torqueconverter 16, then the first speed ratio may be set at 0.2, for example.If the second rotational speed sensor 53 is a rotational speed sensorthat senses the rotational speed of the output shaft of the transmission17, then the first speed ratio may be the product of multiplying thereduction ratio of the transmission 17 by 0.2, for example.

The drive torque exerted on the steering clutches 12L and 12R when thegear sensed by the gear sensor 51 is first gear, the speed ratiocomputed by the speed ratio computer 61 is a first speed ratio, and thevehicle speed is a first speed will be equal to or less than the clutchcapacity of the steering clutches 12L and 12R when the above-mentionedfirst pressure is applied to the steering clutches 12L and 12R. The“clutch capacity” is the amount of torque that the clutches cantransmit. Therefore, if the stall state determination component 60determines that there is no stall state, no slippage occurs at thesteering clutches 12L and 12R. The clutch capacity Tb of the steeringclutches is calculated from the following Formula 1.

Tb=μ×P×A×R×N  (1)

μ: coefficient of friction of friction material constituting steeringclutchP: hydraulic pressure of steering clutch at given timeA: clutch disk surface areaR: clutch disk average pressing diameterN: number of surfaces of clutch disks (2×number of clutches)

The steering clutch controller 70 controls the clutch pressure of thesteering clutches 12L and 12R. If the stall state determinationcomponent 60 determines that the sprockets 2L and 2R are not in a stallstate, the steering clutch controller 70 controls the clutch pressure ofthe steering clutches 12L and 12R to be a first pressure, which is apressure equal to or greater than the holding pressure. Morespecifically, the steering clutch controller 70 controls the steeringclutch control valves 27L and 27R so that the clutch pressure of thesteering clutches 12L and 12R will be the first pressure.

If the stall state determination component 60 determines that one orboth of the sprockets 2L and 2R are in a stall state, the clutchpressure of the steering clutches 12L and 12R corresponding to thesprockets 2L and 2R determined to be in a stall state is raised to asecond pressure that is higher than the above-mentioned first pressure.

In this embodiment, if it is determined that the sprockets 2L and 2R areboth in a stall state, the steering clutch controller 70 raises theclutch pressure of the steering clutches 12L and 12R to the secondpressure. The steering clutch controller 70 controls the steering clutchcontrol valves 27L and 27R so that the clutch pressure of the steeringclutches 12L and 12R will be the second pressure.

The clutch capacity of the steering clutches 12L and 12R when the secondpressure is applied to the steering clutches 12L and 12R is preferablyequal to or greater than the maximum drive torque outputted from thepower transmission device 11. This will effectively prevent slippagefrom occurring in the steering clutches 12L and 12R even if one or bothof the crawler belts are subjected to a high load.

A method for controlling the steering clutches 12L and 12R will now bedescribed through reference to a flowchart. FIG. 28 is a flowchart ofthis method for controlling the steering clutches. FIG. 29 is aflowchart of a method for determining a stall state in the firstembodiment.

First, in step 1, the stall state determination component 60 determineswhether or not one or both of the left and right sprockets 2L and 2R arein a stall state. In this step, in step 11 in FIG. 29, the firstrotational speed sensor 52 senses the first rotational speed, which isthe rotational speed of the input shaft of the power transmission device11. In step 12 the second rotational speed sensor 53 senses the secondrotational speed, which is the rotational speed of the output shaft ofthe power transmission device 11. In step 13 the speed ratio computer 61computes the speed ratio, which is the ratio of the second rotationalspeed to the first rotational speed. In step 14 the vehicle speed sensor62 senses the vehicle speed of the bulldozer 1. In step 15 the gearsensor 51 senses the gear.

In step 16 the stall state determination component 60 determines whetheror not the gear sensed by the gear sensor 51 is first gear, the speedratio computed by the speed ratio computer 61 is equal to or less than afirst speed ratio, and the vehicle speed is equal to or less than aspecific first speed. If the gear sensed by the gear sensor 51 is firstgear, the speed ratio computed by the speed ratio computer 61 is equalto or less than the first speed ratio, and the vehicle speed is equal toor less than the specific first speed (Yes in step 16), then the stallstate determination component 60 determines that the sprockets 2L and 2Rare on a stall state (step 17).

If the gear is second gear or higher, or the speed ratio is higher thanthe first speed ratio, or the vehicle speed is greater than the firstspeed (No in step 16), then the stall state determination component 60determines that the sprockets 2L and 2R are in a non-stall state (step18).

If the stall state determination component 60 determines that there is astall state (Yes in step 1 in FIG. 28), then the steering clutchcontroller 70 raises the clutch pressure of the steering clutches 12Land 12R corresponding to the sprockets 2L and 2R determined to be in astall state to the second pressure (step 3). Upon completion of step 3,the flow returns to step 1. If the stall state determination component60 determines that there is a non-stall state (No in step 1 in FIG. 28),then the steering clutch controller 70 holds the clutch pressure of thesteering clutches 12L and 12R at the first pressure or changes it to thefirst pressure (step 4). Upon completion of step 4, the flow returns tostep 1.

Second Embodiment

The method for determining a stall state is not limited to the method inthe above embodiment, and there are other methods. In the secondembodiment, a stall state determination component 60 a that differs fromthe stall state determination component 60 in the first embodiment willbe described. In the second embodiment, components other than the stallstate determination component 60 a are all the same as in the firstembodiment, and will therefore not be described again.

FIG. 30 is a block diagram of the detailed configuration of a controller30 a according to the second embodiment. The controller 30 a has thestall state determination component 60 a and the steering clutchcontroller 70. Just as in the first embodiment, typically programs anddata for executing the various functions of the stall statedetermination component 60 a and the steering clutch controller 70 arestored in a memory device. When a computation device executes theprogram, the controller 30 a executes the various functions of the stallstate determination component 60 a and the steering clutch controller70. The controller 30 a may also be realized by an integrated circuit.

The stall state determination component 60 a includes a speeddifferential computer 63. A third rotational speed is inputted from athird rotational speed sensor 54, and a fourth rotational speed isinputted from a fourth rotational speed sensor 55 to the speeddifferential computer 63.

The third rotational speed sensor 54 senses the third rotational speed,which is the rotational speed of input-side rotary members of thesteering clutches 12L and 12R. These input-side rotary members are, forexample, clutch disks that link to the above-mentioned lateral shaft 20.The third rotational speed sensor 54 is the rotational speed sensor 41that senses the rotational speed of the lateral shaft 20 (see FIG. 26).

The fourth rotational speed sensor 55 senses the fourth rotationalspeed, which is the rotational speed of output-side rotary members ofthe steering clutches 12L and 12R. These output-side rotary members are,for example, clutch disks that link to the above-mentioned finalreduction gears 22L and 22R. The fourth rotational speed sensor 55 is,for example, rotational speed sensors 42L and 42R that sense therotational speed of the output shafts of the steering clutches 12L and12R (see FIG. 26).

The speed differential computer 63 computes the speed differentialbetween the third rotational speed and the fourth rotational speed atthe steering clutches 12L and 12R. When the speed differential isgreater than a specific threshold in one or both of the steeringclutches 12L and 12R, the stall state determination component 60 adetermines the sprockets 2L and 2R corresponding to the steeringclutches 12L and 12R with a speed differential above the specificthreshold to be in a stall state.

In the second embodiment, the stall state determination component 60 adetermines whether or not the sprockets 2L and 2R are each in a stallstate. The term “stall state” means that the steering clutches 12L and12R are slipping. Therefore, the specific threshold should be one thatallows a determination of whether or not the steering clutches 12L and12R are slipping. Consequently, the specific threshold should be a valuethat is greater than the range of error of the rotation sensors.

The method for controlling the steering clutches 12L and 12R will now bedescribed through reference to a flowchart. FIG. 31 is a flowchart ofthe method for determining a stall state in the second embodiment.

First, in step 21 the third rotational speed sensor 54 senses the thirdrotational speed, which is the rotational speed of the input-side rotarymembers of the steering clutches 12L and 12R. In step 22 the fourthrotational speed sensor 55 senses the fourth rotational speed, which isthe rotational speed of the output-side rotary members of the left andright steering clutches 12L and 12R. In step 23 the speed differentialcomputer 63 computes the speed differential between the third rotationalspeed and the fourth rotational speed for each of the steering clutches12L and 12R.

In step 24 the stall state determination component 60 a determineswhether the speed differential is greater than a specific threshold forone or both of the steering clutches 12L and 12R. If the speeddifferential is greater than the specific threshold at the left steeringclutch 12L (Yes in step 24), the stall state determination component 60a determines the left sprocket 2L to be in a stall state (step 25).Similarly, if the speed differential is greater than the specificthreshold at the right steering clutch 12R (Yes in step 24), the stallstate determination component 60 a determines the right sprocket 2R tobe in a stall state.

If the speed differential is equal to or less than the specificthreshold at the left steering clutch 12L (No in FIG. 24), the stallstate determination component 60 a determines the left sprocket 2L to bein a non-stall state (step 26). Similarly, if the speed differential isequal to or less than the specific threshold at the right steeringclutch 12R (No in step 24), the stall state determination component 60 adetermines the right sprocket 2R to be in a non-stall state (step 26).

In the second embodiment, the steering clutch controller 70 may raisethe clutch pressure to the second pressure only for the steering clutchcorresponding to the sprocket or sprockets determined by the stall statedetermination component 60 a to be in a stall state.

Features

(1) With the bulldozer 1 according to the first and second embodiments,if it is determined that one or both of the sprockets (drive wheels) 2Land 2R are in a stall state, the clutch pressure of the steering clutchor clutches corresponding to one or both sprockets 2L and 2R is raisedto a second pressure that is higher than the first pressure, which isthe normal engagement pressure. Therefore, the clutches are less likelyto slip when the crawler belts are subjected to a high load. As aresult, even if the crawler belts are subjected to a high load, the workcan be carried out the same as usual, which avoids a decrease in workefficiency.

(2) In the first and second embodiments, the clutch capacity of thesteering clutches 12L and 12R when the steering clutches 12L and 12R aresubjected to the second pressure is equal to or greater than the maximumdrive torque outputted from the power transmission device 11. Therefore,this effectively prevents slippage from occurring at the steeringclutches 12L and 12R.

(3) In the first and second embodiments, the clutch pressure of thesteering clutches 12L and 12R rises in proportion to the pressure of thehydraulic fluid supplied to the steering clutches 12L and 12R (hydraulicclutches). This allows the clutch pressure to be flexibly varied for thesteering clutches 12L and 12R with a simple structure.

(4) In the first and second embodiments, the power transmission device11 includes the torque converter 16 that transmits power through afluid. Consequently, when the steering clutches 12L and 12R aresubjected to a load, part of the output of the engine 10 is absorbed bythe torque converter 16, making it less likely that an excessive loadwill be exerted on the steering clutches 12L and 12R.

(5) The stall state determination component 60 according to the firstembodiment determines the sprockets (drive wheels) 2L and 2R both to bein a stall state when the gear sensed by the gear sensor 51 is firstgear, the speed ratio is equal to or less than a first speed ratio, andthe vehicle speed is equal to or less than a specific first speed. Whenthe gear is first gear, the speed ratio is equal to or less than thefirst speed ratio, and the vehicle speed is equal to or less than thespecific first speed, the tractive power of the bulldozer 1 is high, anda high torque is exerted on the steering clutches 12L and 12R.Therefore, the stall state determination component 60 can sense a statein which the steering clutches 12L and 12R are prone to slipping.

(6) The drive torque exerted on the steering clutches 12L and 12R whenthe gear is first gear, the speed ratio is the first speed ratio, andthe vehicle speed is the first speed is equal to or less than the clutchcapacity of the steering clutches 12L and 12R when the first pressure isapplied to the steering clutches 12L and 12R. Therefore, when the stallstate determination component 60 determines that there is no stallstate, the steering clutches 12L and 12R will not slip.

(7) The stall state determination component 60 a according to the secondembodiment determines one or both of the sprockets (drive wheels) 2L and2R to be in a stall state when the speed differential computed by thespeed differential computer 63 for one or both sprockets 2L and 2R isgreater than a specific threshold. Therefore, the stall statedetermination component 60 can detect a state in which the steeringclutches 12L and 12R are slipping.

Modification Examples

The present invention is not limited to or by the above embodiments, andvarious modifications and variations are possible without departing fromthe scope of the present invention.

(a) In the above embodiments, the steering clutch controller 70 raisesthe clutch pressure of the steering clutches 12L and 12R from the firstpressure to the second pressure when a stall state is determined.However, the steering clutch controller 70 may instead raise the clutchpressure of the steering clutches 12L and 12R in stages from the firstpressure to the second pressure. Also, in the second embodiment, thesteering clutch controller 70 may raise the clutch pressure of thesteering clutches 12L and 12R according to the speed differential.

(b) The steering clutches 12L and 12R may be something other thanhydraulic clutches, such as electromagnetic clutches.

(c) In the above embodiments, a bulldozer was used as an example of atracked work vehicle, but the present invention can similarly be appliedto another tracked work vehicle that is equipped with steering clutches.

INDUSTRIAL APPLICABILITY

(A) There are disclosed a work vehicle having steering clutches that canbe hydraulically engaged, wherein the PV value of the sealing rings doesnot reach the maximum permissible value, and a method for controllingthe work vehicle.

(B) There are disclosed a monitoring system for a work vehicle, withwhich a worker can be notified about sealing ring replacement and thelike as needed, according to the usage state of the sealing rings of awork vehicle having hydraulic clutches that can be hydraulicallyengaged, and a work vehicle with which the notification is possible.

(C) There is disclosed a tracked work vehicle with which it is lesslikely that there will be a drop in work efficiency when one or bothcrawler belts are subjected to a high load.

1. A work vehicle, comprising: an engine; a transmission arranged tochange a speed of rotary motion of the engine; a steering clutch thattransmits or cuts off power from the transmission, the steering clutchbeing engaged when supplied with hydraulic fluid that is underengagement pressure; a rotary member having a first hydraulic fluidsupply channel arranged to supply the hydraulic fluid to the steeringclutch, the rotary member being rotated by power from the transmissionwhen the steering clutch is engaged; a drive unit driven by the rotarymember; a support member having a second hydraulic fluid supply channelarranged to supply the hydraulic fluid to the first hydraulic fluidsupply channel, the support member rotatably supporting the rotarymember; a sealing ring disposed between the rotary member and thesupport member, the sealing ring being mounted adjacent to a connectedpart between the first hydraulic fluid supply channel and the secondhydraulic fluid supply channel; a pressure controller that controlspressure of the hydraulic fluid supplied to the steering clutch; and avehicle speed determination component that determines whether or not avehicle speed is at least a specific speed, the pressure controllercontrolling the engagement pressure to become a specific first pressurewhen the vehicle speed is determined to be below the specific speed, andperforming control to reduce the engagement pressure from the firstpressure when the vehicle speed is determined to be at least thespecific speed.
 2. The work vehicle according to claim 1, furthercomprising: a rotational speed sensor measuring a rotational speed ofthe rotary member, the vehicle speed determination component determiningthe vehicle speed to be at least the specific speed when the rotationalspeed measured by the rotational speed sensor is greater than a specificrotational speed threshold.
 3. The work vehicle according to claim 2,wherein the pressure controller performs control to reduce the pressureso that the product of the rotational speed and the pressure of thehydraulic fluid does not exceed a specific upper limit when therotational speed measured by the rotational speed sensor is greater thanthe rotational speed threshold.
 4. The work vehicle according to claim3, wherein the pressure controller performs control to set the pressureof the hydraulic fluid to be the quotient of dividing a first product,which is the product of the first pressure and the rotational speedthreshold, by the rotational speed when the rotational speed is greaterthan the rotational speed threshold.
 5. The work vehicle according toclaim 1, further comprising: a gear sensor that senses a gear of thepower transmission device set by operator input, the vehicle speeddetermination component determining the vehicle speed to be at least thespecific speed when the gear is a first specific gear or a gear higherthan the first specific gear.
 6. The work vehicle according to claim 5,wherein the pressure controller performs control so that the pressure ofthe hydraulic fluid is set to a second pressure that is lower than thefirst pressure, when the gear is the first specific gear or a gearhigher than the first specific gear.
 7. The work vehicle according toclaim 1, wherein the sealing ring seals a gap between the rotary memberand the support member, and the connected part is formed by the gapsealed by the sealing ring.
 8. A work vehicle monitoring systemcomprising: a work vehicle; and a monitoring device provided on anexterior of the work vehicle, the monitoring device being capable ofinputting information from the work vehicle, the work vehicle includingan engine; a transmission arranged to change a speed of rotary motion ofthe engine, a steering clutch that transmits or cuts off power from thetransmission, the steering clutch being engaged when supplied withhydraulic fluid, a rotary member having a first hydraulic fluid supplychannel arranged to supply the hydraulic fluid to the steering clutch,the rotary member being rotated by power from the transmission when thesteering clutch is engaged, a drive unit driven by the rotary member, asupport member having a second hydraulic fluid supply channel arrangedto supply the hydraulic fluid to the first hydraulic fluid supplychannel, the support member rotatably supporting the rotary member, asealing ring disposed between the rotary member and the support member,the sealing ring being mounted adjacent to a connected part between thefirst hydraulic fluid supply channel and the second hydraulic fluidsupply channel, a rotational speed sensor that senses a rotational speedof the rotary member, a controller that decides the pressure of thehydraulic fluid inside the first hydraulic fluid supply channel and thesecond hydraulic fluid supply channel, and performs control so that thepressure of the hydraulic fluid becomes the decided pressure, and anexternal output component that outputs determination basis data relatedto the decided pressure, the rotational speed, and a time when thepressure is controlled by the decided pressure while the rotary memberis rotating, in a format that can be inputted by the monitoring device,and the monitoring device including an output component that outputsmaintenance information about the sealing ring when a predicted wearamount of the sealing ring obtained from the determination basis dataexceeds a specific threshold.
 9. A work vehicle capable of outputtinginformation to an external monitoring device, the work vehiclecomprising: an engine; a transmission arranged to change a speed ofrotary motion of the engine; a steering clutch that transmits or cutsoff power from the transmission, the steering clutch being engaged whensupplied with hydraulic fluid; a rotary member having a first hydraulicfluid supply channel arranged to supply the hydraulic fluid to thesteering clutch, the rotary member being rotated by power from thetransmission when the steering clutch is engaged; a drive unit driven bythe rotary member; a support member having a second hydraulic fluidsupply channel arranged to supply the hydraulic fluid to the firsthydraulic fluid supply channel, the support member rotatably supportingthe rotary member; a sealing ring disposed between the rotary member andthe support member, the sealing ring being mounted adjacent to aconnected part between the first hydraulic fluid supply channel and thesecond hydraulic fluid supply channel; a rotational speed sensor thatsenses a rotational speed of the rotary member; a controller thatdecides a pressure of the hydraulic fluid inside the first hydraulicfluid supply channel and the second hydraulic fluid supply channel, andperforms control so that the pressure of the hydraulic fluid becomes thedecided pressure; and an external output component that outputsdetermination basis data related to the decided pressure, the rotationalspeed, and a time when the pressure is controlled by the decidedpressure while the rotary member is rotating, in a format that can beinputted by the monitoring device.
 10. A work vehicle, comprising: anengine; a transmission arranged to change a speed of rotary motion ofthe engine; a steering clutch that transmits or cuts off power from thetransmission, the steering clutch being engaged when supplied withhydraulic fluid; a rotary member having a first hydraulic fluid supplychannel arranged to supply the hydraulic fluid to the steering clutch,the rotary member being rotated by power from the transmission when thesteering clutch is engaged; a drive unit driven by the rotary member; asupport member having a second hydraulic fluid supply channel arrangedto supply the hydraulic fluid to the first hydraulic fluid supplychannel, the support member rotatably supporting the rotary member; asealing ring disposed between the rotary member and the support member,the sealing ring being mounted adjacent to a connected part between thefirst hydraulic fluid supply channel and the second hydraulic fluidsupply channel; a rotational speed sensor that senses a rotational speedof the rotary member; and a controller that decides a pressure of thehydraulic fluid inside the first hydraulic fluid supply channel and thesecond hydraulic fluid supply channel, and performs control so that thepressure of the hydraulic fluid becomes the decided pressure, thecontroller including an information output component that outputsmaintenance information about the sealing ring when a predicted wearamount of the sealing ring, obtained from the decided pressure, therotational speed, and a time when the pressure is controlled by thedecided pressure while the rotary member is rotating, exceeds a specificthreshold.
 11. The work vehicle according to claim 9, wherein thedetermination basis data is a value obtained by integration of a productof the decided pressure and the rotational speed at a same time as thedecided pressure.
 12. The work vehicle according to claim 9, wherein theexternal output component outputs the determination basis data to acommunication line or to a removable storage medium that can be writtento by the work vehicle and that can be read by the monitoring device.13. The work vehicle according to claim 9, wherein the controllerperforms control to set the pressure to a specific first pressure whenthe rotational speed measured by the rotational speed sensor is no morethan a specific rotational speed threshold, and performs control toreduce the pressure from the first pressure so that a product of therotational speed and the pressure of the hydraulic fluid will not exceeda specific upper limit when the rotational speed measured by therotational speed sensor is greater than a specific rotational speedthreshold.
 14. The work vehicle according to claim 13, wherein thecontroller performs control to set the pressure of the hydraulic fluidto be the quotient of dividing a first product, which is a product ofthe first pressure and the rotational speed threshold, by the rotationalspeed when the rotational speed is greater than the rotational speedthreshold.
 15. The work vehicle according to claim 9, wherein thesealing ring seals a gap between the rotary member and the supportmember, and the connected part is formed by the gap sealed by thesealing ring.
 16. A tracked work vehicle comprising: an engine; left andright drive units respectively having left and right crawler belts andleft and right drive wheels arranged to drive the left and right crawlerbelts; a power transmission device that transmits a power of the engineto the left and right drive wheels; left and right steering clutchesrespectively disposed between the power transmission device and the leftand right drive wheels, the left and right steering clutchestransmitting or cutting off power; left and right steering brakesrespectively disposed between the left and right steering clutches andthe left and right drive wheels, the left and right steering brakesrespectively braking rotation to the left and right drive wheels; astall state determination component that determines whether or not theleft and/or right drive wheel is in a stall state; and a steering clutchcontroller that controls clutch pressures of the left and right steeringclutches, the steering clutch controller controlling the clutchpressures of the left and right steering clutches to be a first pressurewhen it is determined that the left and right drive wheels are not in astall state, and if the left and/or right drive wheel is determined tobe in a stall state, the clutch pressure of the steering clutchcorresponding to the drive wheel determined to be in a stall state israised to a second pressure that is higher than the first pressure. 17.The tracked work vehicle according to claim 16, further comprising: agear sensor that senses a gear of the power transmission device set bythe operator; a first rotational speed sensor that senses a firstrotational speed of an input shaft of the power transmission device; anda second rotational speed sensor that senses a second rotational speedof an output shaft of the power transmission device, the stall statedetermination component including a speed ratio computer that computes aspeed ratio of the second rotational speed to the first rotationalspeed, and a vehicle speed sensor that senses a speed of the trackedwork vehicle, and the stall state determination component determiningthe left and right drive wheels to be in a stall state when the gearsensed by the gear sensor is first gear, the speed ratio is no more thana first speed ratio, and the vehicle speed is no more than a specificfirst speed.
 18. The tracked work vehicle according to claim 17, whereina drive torque exerted on the steering clutch when the gear sensed bythe gear sensor is first gear, the speed ratio is the first speed ratio,and the vehicle speed is the first speed, is no more than a clutchcapacity of the steering clutch when the first pressure is exerted onthe steering clutch.
 19. The tracked work vehicle according to claim 16,further comprising: a third rotational speed sensor that senses a thirdrotational speed of an input-side rotary members of the left and rightsteering clutches; and a fourth rotational speed sensor that senses afourth rotational speed of an output-side rotary members of the left andright steering clutches, the stall state determination componentincluding a speed differential computer that computes a speeddifferential between the third rotational speed and the fourthrotational speed for each of the left and right steering clutches, andwhen the speed differential computed by the speed differential computerfor the left and/or right steering clutch is greater than a specificthreshold, the stall state determination component determines the drivewheels corresponding to the steering clutch whose speed differential isgreater than the threshold to be in a stall state.
 20. The tracked workvehicle according to claim 16, wherein the clutch capacity of thesteering clutch when the second pressure is exerted on the steeringclutch is at least a maximum drive torque outputted from the powertransmission device.
 21. The tracked work vehicle according to claim 16,wherein the steering clutch is engaged when the clutch pressure of thesteering clutch is the first pressure.
 22. The tracked work vehicleaccording to claim 16, wherein the steering clutch is a hydraulicclutch, and the higher the pressure of the hydraulic fluid supplied tothe steering clutch is, the more the clutch pressure rises.